Makefile: reduce repetitive library paths

Makefile: reduce repetitive library paths

When we take a library package we depend on (e.g., LIBPCRE) from a
directory other than the default location of the system, we add the
same directory twice on the linker command like, like so:

  EXTLIBS += -L$(LIBPCREDIR)/$(lib) $(CC_LD_DYNPATH)$(LIBPCREDIR)/$(lib)

Introduce a template "libpath_template" that takes the path to the
directory, which can be used like so:

  EXTLIBS += $(call libpath_template,$(LIBPCREDIR)/$(lib))

and expand it into the "-L$(DIR) $(CC_LD_DYNPATH)$(DIR)" form.
Hopefully we can reduce the chance of typoes this way.

Signed-off-by: Junio C Hamano <gitster@pobox.com>