automake: rework the EGL build

Simplify things by merging the two makefiles. This way we can combine
the duplicated HAVE_PLATFORM_ checks, and build the library without
having a separate static library.

v2: use $() when referencing variables, use correct define (Matt)

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
5 files changed