Bug 236592

In some systems the OpenGL library cannot be found if the pkg-config module is called “opengl” (and henceforth values picked from “opengl.pc”) instead of plainly “gl” (with values from “gl.pc”). One such case are systems where libglvnd (https://github.com/NVIDIA/libglvnd) is used as main GL library, letting it dispatch to the most suitable OpenGL implementation at runtime depending on the system state. In this case the main library would be libOpenGL.so.0 instead of libGL.so.1 and/or libGLX.so.0; then libOpenGL.so.0 will handle dispatching lib(EGL|GLX)_<vendor>.so.<N> (for example libEGL_mesa.so.0 for non-X11 operation with free software Mesa drivers, or libGLX_nvidia.so.0 when running under X with the proprietary Nvidia driver). Some notes: - If opengl.pc is found, the library name will be libOpenGL.so.0, and it will *not* contain GLX symbols. In that case we need to search for glx.pc as well, which may include additional flags/libraries that one needs to link in order to get the GLX symbols. - If gl.pc is found, it can either be provided by libglvnd or not; and in both cases linking to the library specified in the pkg-config module *will* result in being able to find the GLX symbols. Therefore: - If opengl.pc is *not* found (no libglvnd), we want to keep our existing logic: try gl.pc first, fall back to checking the “gl” and “GL” library names, and try to figure out whether GLX headers can be found. - If opengl.pc is found (libglvnd present), we should prefer that as the GL library to link against, and we need to check for the existence of glx.pc to determine whether GLX_FOUND should be set. - If opengl.pc is found but glx.pc is *not* found we might want to fallback to the checking for the gl.pc and the GLX headers as stated above. Thanks for Haelwenn “lanodan” Monnier who pointed out on IRC that Gentoo has been carrying a patch to change the checked pkg-config module and library names to use libglvnd, which allows them to successfully build WebKitGTK :]