FindOpenGL: look for GLVND libraries with a libglvnd suffix

On CentOS 6.10, the libglvnd package from EPEL installs its libraries
under a libglvnd subdirectory.

Modules/FindOpenGL.cmake

@@ -205,11 +205,13 @@ else()
   find_library(OPENGL_glx_LIBRARY
     NAMES GLX
     PATHS ${_OPENGL_LIB_PATH}
+    PATH_SUFFIXES libglvnd
   )
 
   find_library(OPENGL_egl_LIBRARY
     NAMES EGL
     PATHS ${_OPENGL_LIB_PATH}
+    PATH_SUFFIXES libglvnd
   )
 
   find_library(OPENGL_glu_LIBRARY
@@ -264,6 +266,7 @@ else()
             /usr/openwin/lib
             /usr/shlib
             ${_OPENGL_LIB_PATH}
+      PATH_SUFFIXES libglvnd
       )
   endif()