cmake: set OpenGL_GL_PREFERENCE as a cached variable that can be modi…

…fied via the command line

This allows the user to set this to LEGACY. This can be useful for
people under Linux using optirun/primusrun with "old" nvidia drivers
that don't support GLVND properly.

CMakeLists.txt

@@ -423,7 +423,8 @@ endif()
 include(CheckLib)
 include(CheckCXXSourceRuns)
 
-set(OpenGL_GL_PREFERENCE GLVND)
+set(OpenGL_GL_PREFERENCE GLVND CACHE STRING "Linux-only: if GLVND, use the vendor-neutral GL libraries (default). If LEGACY, use the legacy ones (might be necessary to have optirun/primusrun work)")
+set_property(CACHE OpenGL_GL_PREFERENCE PROPERTY STRINGS GLVND LEGACY)
 find_package(OpenGL)
 if (OPENGL_GL)
   include_directories(${OPENGL_INCLUDE_DIR})