configure: allow user to prevent $PATH "sanitization" on Solaris

configure: allow user to prevent $PATH "sanitization" on Solaris

On a Solaris 10 system with Solaris make installed as '/usr/xpg4/bin/make',
GNU make installed as '/usr/local/bin/make', and with '/usr/local/bin'
appearing in $PATH *before* '/usr/xpg4/bin', I was seeing errors like this
upon invoking "make all":

    Usage : make [ -f makefile ][ -K statefile ]...
    make: Fatal error: Unknown option `-C'

This happenes because the Git's Makefile, when running on Solaris,
automatically "sanitizes" $PATH by prepending '/usr/xpg6/bin' and
'/usr/xpg4/bin' to it in order to avoid using non-POSIX /bin/sh from
being used.  In the setup described above, however, this has an
unintended consequence of forcing the use of Solaris make in recursive
make invocations -- even if the $(MAKE) macro is being correctly used in
them!

When building without using the autoconf machinery, this can be solved
by overriding $(SANE_TOOL_PATH).  Teach the autoconf machinery to also
allow users of ./configure to override it from the command line with a
new --with-sane-tool-path option.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>