Evas GL common: Skip shaders generation if there is no change

Evas GL common: Skip shaders generation if there is no change

Based on a quick git diff we check that the glsl code has not changed.
This should fix out-of-tree builds and avoid all source modification
unless required.

When compiling from a tarball there should be no git tree (err 129),
or if there's one the files should not be checked in (ie. no diff).
If you changed the glsl files in a tarball... too bad for you.

If this is still not enough to fix the build, then go ahead and disable
the script from Makefile_Evas.am

I would like to note that the auto-generation during make is extremely
useful when working on the shaders, which is why I'd rather keep it enabled.

@fix