Makefile: introduce SANE_TOOL_PATH for prepending required elements to PATH

Makefile: introduce SANE_TOOL_PATH for prepending required elements to PATH

Some platforms (like SunOS and family) have kept their common binaries at
some historical moment in time, and introduced new binaries with modern
features in a special location like /usr/xpg4/bin or /usr/ucb.  Some of the
features provided by these modern binaries are expected and required by git.
If the featureful binaries are not in the users path, then git could end up
using the less featureful binary and fail.

So provide a mechanism to prepend elements to the users PATH at runtime so
the modern binaries will be found.

Signed-off-by: Brandon Casey <drafnel@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>