Provide missing GL_DEBUG definitions (fix compile on Mac)

[Birch-san]

Apparently we don't require the glext.h header on Mac, but if that's the case, I am not sure why I get compile error:

/Users/birch/git/openage/libopenage/gui/guisys/private/opengl_debug_logger.cpp:15:18: error: use of undeclared identifier
      'GL_DEBUG_CALLBACK_FUNCTION'
                        glGetPointerv(GL_DEBUG_CALLBACK_FUNCTION, &params.callback);
                                      ^
/Users/birch/git/openage/libopenage/gui/guisys/private/opengl_debug_logger.cpp:16:37: error: use of undeclared identifier
      'GL_DEBUG_OUTPUT_SYNCHRONOUS'
                        params.synchronous = glIsEnabled(GL_DEBUG_OUTPUT_SYNCHRONOUS);
                                                         ^
/Users/birch/git/openage/libopenage/gui/guisys/private/opengl_debug_logger.cpp:29:51: error: use of undeclared identifier
      'GL_DEBUG_TYPE_ERROR'
                        functions->glDebugMessageControl(GL_DONT_CARE, GL_DEBUG_TYPE_ERROR, GL_DONT_CARE, 0, nullptr, GL_TRUE);
                                                                       ^
/Users/birch/git/openage/libopenage/gui/guisys/private/opengl_debug_logger.cpp:30:51: error: use of undeclared identifier
      'GL_DEBUG_TYPE_UNDEFINED_BEHAVIOR'
                        functions->glDebugMessageControl(GL_DONT_CARE, GL_DEBUG_TYPE_UNDEFINED_BEHAVIOR, GL_DONT_CARE, 0, nullptr, GL_TRUE);
                                                                       ^
/Users/birch/git/openage/libopenage/gui/guisys/private/opengl_debug_logger.cpp:35:14: error: use of undeclared identifier
      'GL_DEBUG_OUTPUT_SYNCHRONOUS'
                                glEnable(GL_DEBUG_OUTPUT_SYNCHRONOUS);

Maybe we get everything except debug stuff?

I'm not sure that there is KHR_debug support in Mac. Apparently you'd need OpenGL 4.3.
Mac only has OpenGL 4.1.

I decided to throw in the missing #defines, to ensure that the code at least compiled.
The license for glext.h seems pretty permissive, so hopefully this is fine.

[Birch-san]

I am running on macOS Sierra 10.12.5 Beta (16F43c). Here's the compiler configuration I used when I encountered the error.

➜  openage git:(gui-gl-context) ./configure                       
./configure is a convenience script:
it creates the build directory,  symlinks it,
and invokes cmake for an out-of-source build.

Nobody is stopping you from skipping ./configure and our Makefile,
and using CMake directly (e.g. when packaging, or using an IDE).
For your convenience, ./configure even prints the direct CMake invocation!

         build_type | Debug
       cxx_compiler | clang++
          cxx_flags | 
   exe_linker_flags | 
     install_prefix | /usr/local
module_linker_flags | 
shared_linker_flags | 

config options:

          backtrace | if_available
gperftools-profiler | if_available
gperftools-tcmalloc | False
            inotify | if_available

bindir:
/Users/anon/git/openage/.bin/clang++-debug-Oauto-sanitize-none/

invocation:
cmake -DCMAKE_BUILD_TYPE=Debug -DCMAKE_CXX_FLAGS='' -DCMAKE_EXE_LINKER_FLAGS='' -DCMAKE_INSTALL_PREFIX=/usr/local -DCMAKE_MODULE_LINKER_FLAGS='' -DCMAKE_SHARED_LINKER_FLAGS='' -DCXX_OPTIMIZATION_LEVEL=auto -DCXX_SANITIZE_FATAL=False -DCXX_SANITIZE_MODE=none -DWANT_BACKTRACE=if_available -DWANT_GPERFTOOLS_PROFILER=if_available -DWANT_GPERFTOOLS_TCMALLOC=False -DWANT_INOTIFY=if_available -- /Users/anon/git/openage

(running cmake...)

 ___  ______ _______ _______ ___
|  _)/ _____|_______|_______|_  |
| | ( (____  _____      _     | |    ___  ____  _____ ____  _____  ____ _____
| |  \____ \|  ___)    | |    | |   / _ \|  _ \| ___ |  _ \(____ |/ _  | ___ |
| |_ _____) ) |        | |   _| |  | |_| | |_| | ____| | | / ___ ( (_| | ____|
|___|______/|_|        |_|  (___|   \___/|  __/|_____)_| |_\_____|\___ |_____)
                                         |_|                     (_____|

Welcome to the SFT technologies computer-aided openage build system!

You have chosen, or been chosen, to attempt the daring task of building openage.
If you have installed all the dependencies that are conveniently listed in
[doc/building.md], this _might_ just work!

If it doesn't, consider reporting the issue/asking for help in #sfttech on freenode.net.

-- Using python interpreter: /usr/local/opt/python3/bin/python3.6
-- Could NOT find Doxygen (missing:  DOXYGEN_EXECUTABLE) 
CMake Warning at buildsystem/doxygen.cmake:50 (message):
  doxygen couldn't be found, you won't be able to generate docs
Call Stack (most recent call first):
  CMakeLists.txt:97 (doxygen_configure)



cpp library
	libopenage                                          [237 sources] [14 generated]
-- Could NOT find GCCBacktrace (missing:  GCCBacktrace_LIBRARIES GCCBacktrace_INCLUDE_DIRS) 
unavailable: backtrace
-- Could NOT find Gperftools (missing:  GPERFTOOLS_LIBRARIES GPERFTOOLS_INCLUDE_DIR) 
unavailable: gperftools-profiler
-- Could NOT find INOTIFY (missing:  INOTIFY_INCLUDE_DIR) 
unavailable: inotify

cython module
	run                                                 [embedded interpreter] [noinstall]
	openage.cabextract.lzxd
	openage.cabextract.cabchecksum                      [standalone]
	openage.convert.slp
	openage.convert.interface.visgrep
	openage.cppinterface.exctranslate
	openage.cppinterface.exctranslate_tests
	openage.cppinterface.pyobject
	openage.cppinterface.setup_checker
	openage.cvar.cvar
	openage.game.main_cpp
	openage.log.log_cpp
	openage.util.filelike.cpp
	openage.util.fslike.cpp
	openage.testing.cpp_testing
	openage.testing.misc_cpp

enabled options:
	<none>

disabled options:
	backtrace
	gperftools-profiler
	gperftools-tcmalloc
	inotify

openage v0.3.0-861-g85e62658

         compiler | AppleClang 8.0.0.8000042
       build type | Debug
         cxxflags |  -Wall -Wextra -pedantic -Wno-mismatched-tags -stdlib=libc++ -std=c++14
 build type flags | -g -O0
        build dir | /Users/anon/git/openage/.bin/clang++-debug-Oauto-sanitize-none
   install prefix | /usr/local
py install prefix | /usr/local/Cellar/python3/3.6.1/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages

-- Configuring done
-- Generating done
-- Build files have been written to: /Users/anon/git/openage/.bin/clang++-debug-Oauto-sanitize-none