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: