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.
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