gallium: Respect user's CFLAGS for including X headers

This can break on systems that don't have a system X installation.

Signed-off-by: Dan Nicholson <dbn.lists@gmail.com>

src/gallium/state_trackers/glx/xlib/Makefile

@@ -5,7 +5,8 @@ LIBNAME = xlib
 
 LIBRARY_INCLUDES = \
 	-I$(TOP)/include \
-	-I$(TOP)/src/mesa
+	-I$(TOP)/src/mesa \
+	$(X_CFLAGS)
 
 C_SOURCES = \
 	glx_api.c \

src/gallium/winsys/xlib/Makefile

@@ -6,7 +6,8 @@ LIBNAME = ws_xlib
 LIBRARY_INCLUDES = \
 	-I$(TOP)/src/gallium/include \
 	-I$(TOP)/src/gallium/drivers \
-	-I$(TOP)/src/gallium/auxiliary
+	-I$(TOP)/src/gallium/auxiliary \
+	$(X_CFLAGS)
 
 C_SOURCES = \
 	xlib_sw_winsys.c