fvwm2: minimal fix for silly configure script attempting to detect -Werror

Configure tests tend to use broken C code and fail with compiler updates.
But one that is deliberately broken to catch -Werror is a new brilliancy.

A more radical approach is https://github.com/fvwmorg/fvwm/pull/106

x11/fvwm2/patches/patch-configure

@@ -1,6 +1,15 @@
 Index: configure
 --- configure.orig
 +++ configure
+@@ -7645,7 +7645,7 @@ main (void)
+ {
+ int i; static j; int *p; char *c;
+   switch (*p = p = *c) { case 0: printf("%Q", c, p); }
+-  *c = &i; c = p;
++  *c = &i;
+   while (1 || (unsigned int)3 >= 0 || ((int)-1) == ((unsigned int)1));
+ 
+   ;
 @@ -12109,7 +12109,7 @@ then :
  else $as_nop