m4: introduce st_SAVE_VAR and st_RESTORE_VAR macros

These new macros are going to be needed for the upcoming
AX_PROG_CC_FOR_BUILD change.

* m4/st_save_restore_var.m4: New file.

diff --git a/m4/st_save_restore_var.m4 b/m4/st_save_restore_var.m4
new file mode 100644 (file)
index 0000000..2939380
--- /dev/null
+++ b/m4/st_save_restore_var.m4
@@ -0,0 +1,7 @@
+AC_DEFUN([st_SAVE_VAR], [dnl
+AS_IF([test -n "${$1+set}"], [st_saved_$1="${$1}"; unset $1])
+])
+
+AC_DEFUN([st_RESTORE_VAR], [dnl
+AS_IF([test -n "${st_saved_$1+set}"], [$1="${st_saved_$1}"; unset st_saved_$1])
+])