build: Only conditionally mention the GL headers in external.

They might be dropped form distro source packages since they're duplicates.

external/include/CMakeLists.txt

@@ -2,9 +2,16 @@
 set( INCLUDE_DIR "${CMAKE_CURRENT_SOURCE_DIR}" )
 
 set( GL_HEADERS
-    ${INCLUDE_DIR}/GL/gl_format.h
-    ${INCLUDE_DIR}/GL/glext.h
-    ${INCLUDE_DIR}/GL/wglext.h )
+    ${INCLUDE_DIR}/GL/gl_format.h )
+
+# These can be dropped by some subsetted source distributions since
+# they ought to exist, if required, in your OpenGL SDK package.
+foreach(FN ${INCLUDE_DIR}/GL/glext.h ${INCLUDE_DIR}/GL/wglext.h)
+    if(EXISTS "${FN}")
+        list(APPEND GL_HEADERS "${FN}")
+    endif()
+endforeach()
+
 set( UTILS_HEADERS
     ${INCLUDE_DIR}/utils/algebra.h
     ${INCLUDE_DIR}/utils/sysinfo.h