CMake: Fix build breakage trying to set OpenGL version

Fixes #660.

src/CMakeLists.txt

@@ -125,6 +125,7 @@ elseif(${PLATFORM} MATCHES "Raspberry Pi")
 endif()
 
 if (${OPENGL_VERSION})
+  MESSAGE(STATUS "Detected OpenGL version: ${OPENGL_VERSION}")
   set(${SUGGESTED_GRAPHICS} "${GRAPHICS}")
   if (${OPENGL_VERSION} MATCHES "3.3")
     set(GRAPHICS "GRAPHICS_API_OPENGL_33")
@@ -135,7 +136,7 @@ if (${OPENGL_VERSION})
   elseif (${OPENGL_VERSION} MATCHES "ES 2.0")
     set(GRAPHICS "GRAPHICS_API_OPENGL_ES2")
   endif()
-  if (${SUGGESTED_GRAPHICS} AND NOT "${SUGGESTED_GRAPHICS}" STREQUAL "${GRAPHICS}")
+  if ("${SUGGESTED_GRAPHICS}" AND NOT "${SUGGESTED_GRAPHICS}" STREQUAL "${GRAPHICS}")
     message(WARNING "You are overriding the suggested GRAPHICS=${SUGGESTED_GRAPHICS} with ${GRAPHICS}! This may fail")
   endif()
 endif()