Move computation of absolute paths from Makefile to runtime (in preparation for RUNTI...

Move computation of absolute paths from Makefile to runtime (in preparation for RUNTIME_PREFIX)

This commit prepares the Makefile for relocatable binaries (called
RUNTIME_PREFIX).  Such binaries will be able to be moved together
with the system configuration files to a different directory,
requiring to compute the prefix at runtime.

In a first step, we make all paths relative in the Makefile and
teach system_path() to add the prefix instead.  We used to compute
absolute paths in the Makefile and passed them to C as defines.  We
now pass relative paths to C and call system_path() to add the
prefix at runtime.

Signed-off-by: Steffen Prohaska <prohaska@zib.de>
Acked-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>