Don't link agains OpenGL libraries when it's not needed

configure.ac

@@ -134,12 +134,8 @@ esac
 
 dnl Checks for libraries.
 
-null_LIBS="$LIBS"
-
 AC_SEARCH_LIBS(cos, m)
 
-base_LIBS="$LIBS"
-
 dnl Thread related checks
 AC_CHECK_HEADER(pthread.h)
 AC_CHECK_LIB(pthread, pthread_exit)
@@ -188,6 +184,9 @@ else
     fi
 fi
 
+base_LIBS="$LIBS"
+LIBS=""
+
 dnl check for OpenGL related libraries
 case "${host}" in
 *-*-cygwin* | *-*-mingw32*)