documentation: Makefile accounts for SHELL_PATH setting
documentation: Makefile accounts for SHELL_PATH setting

Ensure that the Makefile that generates and installs the Documentation is
aware of any SHELL_PATH setting.  Use this value if found or the current
setting for SHELL if not.  This is an accommodation for systems where sh
is not POSIX enough.

Signed-off-by: Ben Walton <6f9de98cc1f3d6d6e7da9953ca2250e07ad38e43@artsci.utoronto.ca>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git