Merge branch 'dj/runtime-prefix'

Merge branch 'dj/runtime-prefix'

A build-time option has been added to allow Git to be told to refer
to its associated files relative to the main binary, in the same
way that has been possible on Windows for quite some time, for
Linux, BSDs and Darwin.

* dj/runtime-prefix:
  Makefile: quote $INSTLIBDIR when passing it to sed
  Makefile: remove unused @@PERLLIBDIR@@ substitution variable
  mingw/msvc: use the new-style RUNTIME_PREFIX helper
  exec_cmd: provide a new-style RUNTIME_PREFIX helper for Windows
  exec_cmd: RUNTIME_PREFIX on some POSIX systems
  Makefile: add Perl runtime prefix support
  Makefile: generate Perl header from template file