libsepol: build: follow standard semantics for DESTDIR and PREFIX

This patch solves the following issues:
- The pkg-config files generates odd paths when using DESTDIR without PREFIX
- DESTDIR is needed during compile time to compute library and header paths which it should not.
- Installing with both DESTDIR and PREFIX set gives us odd paths
- Make usage of DESTDIR and PREFIX more standard

Signed-off-by: Marcus Folkesson <marcus.folkesson@gmail.com>
6 files changed