There is no glesv3.pc provided by mesa, perhaps because
the glesv3 support is provided by the libGLESv2 library.
Don't bother testing for glesv3.pc, just check for glesv2.pc
and then search for the gl3.h header file.

This fixes an issue reported by Theo Veenker, where building
with glesv3 enabled would result in a cairo.pc file that depends
on the non-existant glesv3.pc.