configure: add check for GNU indent

configure: add check for GNU indent

Only GNU indent is supported when indenting autogenerated format_pack.c
and format_unpack.c files. Some non-GNU indent (Mac OS X and FreeBSD)
add extra whitespaces than break the build of those files.

Fallback to 'cat' if a non-GNU indent is found.

Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=88335
Signed-off-by: Samuel Iglesias Gonsalvez <siglesias@igalia.com>
Tested-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>