Added a check for a scanf that knows about %as.

m4/gnuscanf.m4

@@ -0,0 +1,25 @@
+dnl Check for a scanf that understands about %as as format specifier
+
+AC_DEFUN(tinc_SCANF_AS,
+[
+  AC_CACHE_CHECK([for a scanf that groks %as], tinc_cv_scanf_as,
+  [
+    AC_TRY_RUN([
+/* Very naive program which will probably give a segmentation
+   fault if the sscanf doesn't work as expected. */
+#include <stdio.h>
+int main() {
+  char*s = NULL;
+  sscanf("string\n", "%as\n", &s);
+  if(s == NULL)
+    return 1;
+  return strcmp("string", s);
+}
+    ], [tinc_cv_scanf_as="yes"], [tinc_cv_scanf_as="no"])
+  ])
+
+if test "$tinc_cv_scanf_as" = "yes" ; then
+  AC_DEFINE(HAVE_SCANF_AS)
+  AC_SUBST(HAVE_SCANF_AS)
+fi
+])