Make.Rules: add gperf detection and fix INDENT test

Move gperf detection from libcap/Makefile to Make.Rules to be more cross-environment friendly.
Fix INDENT test (dollar sign must be doubled):
http://www.gnu.org/software/make/manual/make.html#Variables-in-Recipes

Signed-off-by: Matthieu Crapet <mcrapet@gmail.com>
Signed-off-by: Andrew G Morgan <morgan@kernel.org>