mesa: Set api prefix to version string when overriding version

mesa: Set api prefix to version string when overriding version

Otherwise there are problems when user overrides version and application
such as Piglit wants to detect used api with glGetString(GL_VERSION).

This makes it currently impossible to run glslparsertest tests for
OpenGL ES when using version override.

Below is example when using MESA_GLES_VERSION_OVERRIDE=3.1.

Before:
	"3.1 Mesa 11.1.0-devel (git-24a1a15)"

After:
	"OpenGL ES 3.1 Mesa 11.1.0-devel (git-78042ff)"

v2: only include api prefix for OpenGL ES (Boyan Ding)

Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Cc: "11.0" <mesa-stable@lists.freedesktop.org>