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.
