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/shared-glapi/SConscript

@@ -0,0 +1,116 @@
+#######################################################################
+# SConscript for shared-glapi/es1api/es2api
+
+from sys import executable as python_cmd
+
+Import('*')
+
+def mapi_objects(env, printer, mode):
+    """Return mapi objects built for the given printer and mode."""
+    mapi_sources = {
+        'glapi': ['entry.c', 'mapi_glapi.c', 'stub.c', 'table.c',
+                  'u_current.c', 'u_execmem.c', 'u_thread.c'],
+        'bridge': ['entry.c'],
+    }
+    mapi_defines = {
+        'glapi': ['MAPI_MODE_GLAPI'],
+        'bridge': ['MAPI_MODE_BRIDGE'],
+    }
+
+    header_name = '%s-tmp.h' % (printer)
+
+    # generate ABI header
+    header = env.CodeGenerate(
+        target = header_name,
+        script = '../mapi/mapi_abi.py',
+        source = '../glapi/gen/gl_and_es_API.xml',
+        command = python_cmd + ' $SCRIPT ' + \
+                '--printer %s --mode lib $SOURCE > $TARGET' % (printer),
+    )
+
+    cpppath = [
+        header[0].dir,
+        '#/include',
+        '#/src/mapi',
+    ]
+    
+    cppdefines = mapi_defines[mode] + [
+        'MAPI_ABI_HEADER=\\"%s\\"' % (header_name),
+    ]
+
+    if env['platform'] == 'windows':
+        if mode == 'glapi':
+            cppdefines += [
+                '_GLAPI_DLL_EXPORTS', # declare _glapi_* as __declspec(dllexport) in glapi.h
+            ]
+        else:
+            cppdefines += [
+                '_GDI32_', # prevent gl* being declared __declspec(dllimport) in MS headers
+                'BUILD_GL32', # declare gl* as __declspec(dllexport) in Mesa headers
+            ]
+
+    objects = []
+    for s in mapi_sources[mode]:
+        o = env.SharedObject(
+            target = '%s-%s' % (printer, s[:-2]),
+            source = '../mapi/' + s,
+            CPPPATH = cpppath,
+            CPPDEFINES = cppdefines,
+        )
+        objects.append(o[0])
+
+    env.Depends(objects, header)
+
+    return objects
+
+if env['platform'] != 'winddk':
+    env = env.Clone()
+
+    env['SHLIBPREFIX'] = 'lib'
+    env['LIBPREFIX'] = 'lib'
+
+    shared_glapi_objects = mapi_objects(env, 'shared-glapi', 'glapi')
+    shared_glapi = env.SharedLibrary(
+        target = 'glapi',
+        source = shared_glapi_objects,
+    )
+
+    # manually add LIBPREFIX on windows
+    if env['platform'] == 'windows':
+        libs = ['libglapi']
+    else:
+        libs = ['glapi']
+
+    es1api_objects = mapi_objects(env, 'es1api', 'bridge')
+    es1api = env.SharedLibrary(
+        target = 'GLESv1_CM',
+        source = es1api_objects,
+        LIBPATH = ['.'],
+        LIBS = libs,
+    )
+
+    es2api_objects = mapi_objects(env, 'es2api', 'bridge')
+    es2api = env.SharedLibrary(
+        target = 'GLESv2',
+        source = es2api_objects,
+        LIBPATH = ['.'],
+        LIBS = libs,
+    )
+
+    env.InstallSharedLibrary(shared_glapi, version=(0, 0, 0))
+    env.InstallSharedLibrary(es1api, version=(1, 0, 0))
+    env.InstallSharedLibrary(es2api, version=(2, 0, 0))
+
+    if env['platform'] == 'windows':
+        shared_glapi = env.FindIxes(shared_glapi, 'LIBPREFIX', 'LIBSUFFIX')
+    else:
+        shared_glapi = env.FindIxes(shared_glapi, 'SHLIBPREFIX', 'SHLIBSUFFIX')
+
+    # build glapi bridge as a convenience libarary for libgl-xlib/libgl-gdi
+    bridge_glapi_objects = mapi_objects(env, 'glapi', 'bridge')
+    bridge_glapi = env.ConvenienceLibrary(
+        target = 'glapi_bridge',
+        source = bridge_glapi_objects,
+    )
+
+    Export(['shared_glapi', 'bridge_glapi'])