configure: Allow GIT_ARG_SET_PATH to handle --without-PROGRAM

Add an optional second argument to both GIT_ARG_SET_PATH and
GIT_CONF_APPEND_PATH such that any value of the second argument will
enable configure to set NO_$PROGRAM in addition to an empty
$PROGRAM_PATH.  This is initially useful for allowing configure to
disable the use of python, as the remote helper code has nothing
leveraging it yet.

The Makefile already recognizes NO_PYTHON, but configure provided no
way to set it appropriately.

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

configure.ac

@@ -23,21 +23,32 @@ AC_DEFUN([GIT_CONF_APPEND_LINE],
 # GIT_ARG_SET_PATH(PROGRAM)
 # -------------------------
 # Provide --with-PROGRAM=PATH option to set PATH to PROGRAM
+# Optional second argument allows setting NO_PROGRAM=YesPlease if
+# --without-PROGRAM version used.
 AC_DEFUN([GIT_ARG_SET_PATH],
 [AC_ARG_WITH([$1],
  [AS_HELP_STRING([--with-$1=PATH],
                  [provide PATH to $1])],
- [GIT_CONF_APPEND_PATH($1)],[])
+ [GIT_CONF_APPEND_PATH($1,$2)],[])
 ])# GIT_ARG_SET_PATH
 #
 # GIT_CONF_APPEND_PATH(PROGRAM)
 # ------------------------------
 # Parse --with-PROGRAM=PATH option to set PROGRAM_PATH=PATH
 # Used by GIT_ARG_SET_PATH(PROGRAM)
+# Optional second argument allows setting NO_PROGRAM=YesPlease if
+# --without-PROGRAM is used.
 AC_DEFUN([GIT_CONF_APPEND_PATH],
 [PROGRAM=m4_toupper($1); \
 if test "$withval" = "no"; then \
-	AC_MSG_ERROR([You cannot use git without $1]); \
+	if test -n "$2"; then \
+		m4_toupper($1)_PATH=$withval; \
+		AC_MSG_NOTICE([Disabling use of ${PROGRAM}]); \
+		GIT_CONF_APPEND_LINE(NO_${PROGRAM}=YesPlease); \
+		GIT_CONF_APPEND_LINE(${PROGRAM}_PATH=); \
+	else \
+		AC_MSG_ERROR([You cannot use git without $1]); \
+	fi; \
 else \
 	if test "$withval" = "yes"; then \
 		AC_MSG_WARN([You should provide path for --with-$1=PATH]); \