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/mapi/glapi/SConscript

@@ -17,6 +17,11 @@ if env['platform'] != 'winddk':
             '_GDI32_', # prevent gl* being declared __declspec(dllimport) in MS headers
             'BUILD_GL32', # declare gl* as __declspec(dllexport) in Mesa headers
         ])
+        if env['gles']:
+            env.Append(CPPDEFINES = ['_GLAPI_DLL_EXPORTS'])
+        else:
+            # prevent _glapi_* from being declared __declspec(dllimport)
+            env.Append(CPPDEFINES = ['_GLAPI_NO_EXPORTS'])
 
     env.Append(CPPPATH = [
         '#/src/mapi',