Force abort if GL-headers are missing. Quick and dirty solution - user might...

Force abort if GL-headers are missing. Quick and dirty solution - user might not want graphical output for ALBERT!

configure.ac

@@ -46,19 +46,20 @@ AC_CHECK_FUNCS([alarm pow sqrt strdup strchr strstr],,
 AC_FUNC_MALLOC
 AC_FUNC_VPRINTF
 
+# look for X11 library paths AND sets all necessary flags (X_CFLAGS) e.g.
+AC_PATH_XTRA
+
 # Checks for header files.
 AC_HEADER_STDC
 AC_CHECK_HEADERS([malloc.h unistd.h X11/Xlib.h X11/Xutil.h],,
 		 [AC_MSG_ERROR([Exiting...])])
-AC_CHECK_HEADERS([GL/gl.h GL/glx.h rpc/xdr.h])
+AC_CHECK_HEADERS([GL/gl.h GL/glx.h rpc/xdr.h],,[AC_MSG_ERROR([Exiting...])])
 
 # Checks for typedefs, structures, and compiler characteristics.
 AC_C_CONST
 AC_TYPE_SIZE_T
 AC_TYPE_SIGNAL
 
-# look for X11 library paths AND sets all necessary flags (X_CFLAGS) e.g.
-AC_PATH_XTRA
 
 # ******************************************************************************
 # These are our own tests