Makefile: simplify output of the libpath_template

Makefile: simplify output of the libpath_template

If a platform lacks the support to specify the dynamic library path,
there is no suitable value to give to the CC_LD_DYNPATH variable.
Allow them to be set to an empty string to signal that they do not
need to add the usual -Wl,-rpath, or -R or whatever option followed
by a directory name.  This way,

    $(call libpath_template,$(SOMELIBDIR))

would expand to just a single mention of that directory, i.e.

    -L$(SOMELIBDIR)

when CC_LD_DYNPATH is set to an empty string (or a "-L", which
would have repeated the same "-L$(SOMELIBDIR)" twice without any
ill effect).

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