premake4 build: allow to dynamically load X11 if X11 system headers/library is missing

premake4 build: allow to dynamically load OpenGL/GLEW/glx if system headers are missing

btgui/OpenGLWindow/optionalX11/X11/XlibConf.h

@@ -0,0 +1,38 @@
+/* include/X11/XlibConf.h.  Generated from XlibConf.h.in by configure.  */
+/*
+ * Copyright © 2005 Keith Packard
+ *
+ * Permission to use, copy, modify, distribute, and sell this software and its
+ * documentation for any purpose is hereby granted without fee, provided that
+ * the above copyright notice appear in all copies and that both that
+ * copyright notice and this permission notice appear in supporting
+ * documentation, and that the name of Keith Packard not be used in
+ * advertising or publicity pertaining to distribution of the software without
+ * specific, written prior permission.  Keith Packard makes no
+ * representations about the suitability of this software for any purpose.  It
+ * is provided "as is" without express or implied warranty.
+ *
+ * KEITH PACKARD DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE,
+ * INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS, IN NO
+ * EVENT SHALL KEITH PACKARD BE LIABLE FOR ANY SPECIAL, INDIRECT OR
+ * CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE,
+ * DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER
+ * TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+ * PERFORMANCE OF THIS SOFTWARE.
+ */
+
+#ifndef _XLIBCONF_H_
+#define _XLIBCONF_H_
+/*
+ * This header file exports defines necessary to correctly
+ * use Xlibint.h both inside Xlib and by external libraries
+ * such as extensions.
+ */
+
+/* Threading support? */
+#define XTHREADS 1
+
+/* Use multi-threaded libc functions? */
+#define XUSE_MTSAFE_API 1
+
+#endif /* _XLIBCONF_H_ */