Makefile: add a target which will abort compilation with ancient shells

Makefile: add a target which will abort compilation with ancient shells

This adds a make target which can be used to try to execute certain shell
constructs which are required for compiling and running git.

This patch provides a test for the $() notation for command substition
which is used in the Makefile and extensively in the git scripts.

The make target is named in such a way as to be a hint to the user that
SHELL_PATH should be set to an appropriate shell. If the shell command
fails, the user should receive a message similar to the following:

make: *** [please_set_SHELL_PATH_to_a_more_modern_shell] Error 2

Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil>
Signed-off-by: Junio C Hamano <gitster@pobox.com>