automake: Check for 'indent' and fall back to 'cat' if not found.

automake: Check for 'indent' and fall back to 'cat' if not found.

The glapi generator code uses indent to produce more readable code.
However, we don't want to make GNU indent a hard build dependency; check
for it in configure.ac and fall back to 'cat' if it's not available.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=50484
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Acked-by: Ben Widawsky <ben@bwidawsk.net>