scons: add code to generate the various GL API files

This fixes recent build breakage when we began building the generated
API files from xml as part of the normal build process.

Fixes http://bugs.freedesktop.org/show_bug.cgi?id=50475

src/gallium/targets/libgl-xlib/SConscript

@@ -35,6 +35,9 @@ sources = [
     'xlib.c',
 ]
 
+# The sources depend on the python-generated GL API files/headers.
+env.Depends(sources, glapi_headers)
+
 if True:
     env.Append(CPPDEFINES = ['GALLIUM_TRACE', 'GALLIUM_RBUG', 'GALLIUM_GALAHAD', 'GALLIUM_SOFTPIPE'])
     env.Prepend(LIBS = [trace, rbug, galahad, softpipe])