Export GLLoadGen symbols so they can be used in shared libraries.

Caused linker problems mainly on Windows, where every symbol would need
to be exported manually, on Linux this was done implicitly (or with
`-fvisibility=default`). I now maintain glLoadGen fork at
https://github.com/mosra/glloadgen.git containing changes needed for
Magnum.

The `ogl_*()` functions are used only internally, various global
variables for extension queries are not used at all and thus they don't
need to be exported. I thus enabled `-fvisibility=hidden`.

Fixes #16.

external/OpenGL/GL/CMakeLists.txt

@@ -23,6 +23,6 @@
 #
 
 add_library(MagnumGLLoadGenObjects OBJECT gl_magnum.c)
-set_target_properties(MagnumGLLoadGenObjects PROPERTIES COMPILE_FLAGS "${CMAKE_SHARED_LIBRARY_CXX_FLAGS}")
+set_target_properties(MagnumGLLoadGenObjects PROPERTIES COMPILE_FLAGS "${CMAKE_SHARED_LIBRARY_CXX_FLAGS} -fvisibility=hidden -DGLLoadGen_EXPORTS")
 
 install(FILES gl_magnum.h DESTINATION ${MAGNUM_INCLUDE_INSTALL_DIR}/OpenGL/GL)

external/OpenGL/GL/README.md

@@ -1,10 +1,6 @@
-OpenGL header and extension loader is generated using [glLoadGen](https://bitbucket.org/alfonse/glloadgen),
-preferrably from tip of the development branch, currently `2.0`. For it to work
-flawlessly on OpenGL 2.1 and simultaenously not containing all that pre-core
-stuff, you need to patch it with included `glloadgen.patch`, which will then
-force it to use pre-3.0 `glGetString(GL_EXTENSIONS)` even if we request OpenGL
-4.4. Otherwise it would use `glGetStringi(GL_EXTENSIONS, i)`, which is not
-available on OpenGL 2.1 and extension loading will fail on these systems.
+OpenGL header and extension loader is generated using glLoadGen, originally
+from [BitBucket](https://bitbucket.org/alfonse/glloadgen), with Magnum-related
+changes at [](https://github.com/mosra/glloadgen.git).
 
 See `extensions.txt` for a list of requested non-core extensions. Copy the file
 into glLoadGen directory and generate the files using Lua: