git_sha1_gen: use git_sha1.h.in on all build systems

git_sha1_gen: use git_sha1.h.in on all build systems

Meson already uses this, let's get the other build sys to use it too.

Note: rstrip() was dropped, as truncating to the first 10 chars already
gets rid of the terminating newline (not an issue with the env var
either, unless maliciously crafted to break the build... not sure this
is a real-world issue).

Verified to work and give the same output as before on both python 2
and 3 :)

Signed-off-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Brian Paul <brianp@vmware.com>