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>

Makefile

@@ -1062,7 +1062,7 @@ export TAR INSTALL DESTDIR SHELL_PATH
 
 SHELL = $(SHELL_PATH)
 
-all:: $(ALL_PROGRAMS) $(BUILT_INS) $(OTHER_PROGRAMS) GIT-BUILD-OPTIONS
+all:: shell_compatibility_test $(ALL_PROGRAMS) $(BUILT_INS) $(OTHER_PROGRAMS) GIT-BUILD-OPTIONS
 ifneq (,$X)
 	$(foreach p,$(patsubst %$X,%,$(filter %$X,$(ALL_PROGRAMS) $(BUILT_INS) git$X)), $(RM) '$p';)
 endif
@@ -1075,6 +1075,11 @@ endif
 	$(QUIET_SUBDIR0)perl $(QUIET_SUBDIR1) PERL_PATH='$(PERL_PATH_SQ)' prefix='$(prefix_SQ)' all
 	$(QUIET_SUBDIR0)templates $(QUIET_SUBDIR1)
 
+please_set_SHELL_PATH_to_a_more_modern_shell:
+	@$$(:)
+
+shell_compatibility_test: please_set_SHELL_PATH_to_a_more_modern_shell
+
 strip: $(PROGRAMS) git$X
 	$(STRIP) $(STRIP_OPTS) $(PROGRAMS) git$X
 
@@ -1457,6 +1462,7 @@ endif
 	$(RM) GIT-VERSION-FILE GIT-CFLAGS GIT-GUI-VARS GIT-BUILD-OPTIONS
 
 .PHONY: all install clean strip
+.PHONY: shell_compatibility_test please_set_SHELL_PATH_to_a_more_modern_shell
 .PHONY: .FORCE-GIT-VERSION-FILE TAGS tags cscope .FORCE-GIT-CFLAGS
 .PHONY: .FORCE-GIT-BUILD-OPTIONS