configure: add macro to set arbitrary make variables

Add macro GIT_PARSE_WITH_SET_MAKE_VAR to configure.ac to allow --with
style options that set values for variables used during the make
process.

Arguments are the $name part of --with-$name, the name of
the variable to set in the Makefile (config.mak.autogen) and
the help text for the option.

Signed-off-by: Ben Walton <bwalton@artsci.utoronto.ca>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

configure.ac

@@ -68,6 +68,26 @@ else \
 	GIT_CONF_APPEND_LINE(${PACKAGE}DIR=$withval); \
 fi \
 ])# GIT_PARSE_WITH
+#
+# GIT_PARSE_WITH_SET_MAKE_VAR(WITHNAME, VAR, HELP_TEXT)
+# ---------------------
+# Set VAR to the value specied by --with-WITHNAME.
+# No verification of arguments is performed, but warnings are issued
+# if either 'yes' or 'no' is specified.
+# HELP_TEXT is presented when --help is called.
+# This is a direct way to allow setting variables in the Makefile.
+AC_DEFUN([GIT_PARSE_WITH_SET_MAKE_VAR],
+[AC_ARG_WITH([$1],
+ [AS_HELP_STRING([--with-$1=VALUE], $3)],
+ if test -n "$withval"; then \
+  if test "$withval" = "yes" -o "$withval" = "no"; then \
+    AC_MSG_WARN([You likely do not want either 'yes' or 'no' as]
+		     [a value for $1 ($2).  Maybe you do...?]); \
+  fi; \
+  \
+  AC_MSG_NOTICE([Setting $2 to $withval]); \
+  GIT_CONF_APPEND_LINE($2=$withval); \
+ fi)])# GIT_PARSE_WITH_SET_MAKE_VAR
 
 dnl
 dnl GIT_CHECK_FUNC(FUNCTION, IFTRUE, IFFALSE)