scons: Add support for GLES.

GLES can be enabled by running scons with

  $ scons gles=yes

When gles=yes is given, the build is changed in three ways.  First,
libmesa.a will be built with FEATURE_ES1 and FEATURE_ES2.  This makes
DRI drivers and libEGL support and advertise GLES support.  Second, GLES
libraries will be created.  They are libGLESv1_CM, libGLESv2, and
libglapi.  Last, libGL or opengl32 will link to libglapi.  This change
is required as _glapi_* will be declared as __declspec(dllimport) in
libmesa.a on windows.  libmesa.a expects those symbols to be defined in
another DLL.  Due to this change to GL, GLES support is marked
experimental.

Note that GLES requires libxml2-python to generate some of its sources.

src/gallium/targets/libgl-xlib/SConscript

@@ -16,6 +16,12 @@ env.Append(CPPDEFINES = ['USE_XSHM'])
 
 env.Prepend(LIBS = env['X11_LIBS'])
 
+# when GLES is enabled, gl* and _glapi_* belong to bridge_glapi and
+# shared_glapi respectively
+if env['gles']:
+    env.Prepend(LIBPATH = [shared_glapi.dir])
+    glapi = [bridge_glapi, 'glapi']
+
 env.Prepend(LIBS = [
     st_xlib,
     ws_xlib,