Explicitly check for libGL before building

Change-Id: I5a84bc396925828a11d3c680d1a52e99321547a1
Reviewed-on: https://gerrit.libreoffice.org/23335Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Michael Stahl <mstahl@redhat.com>

configure.ac

@@ -10438,6 +10438,7 @@ elif test $_os = WINNT; then
 else
     if test "$USING_X11" = TRUE; then
         ENABLE_OPENGL=TRUE
+        AC_CHECK_LIB(GL, glBegin, [:], AC_MSG_ERROR([libGL required.]))
     fi
     ENABLE_OPENGL_CANVAS=TRUE
 fi