build: relax the relaxed stack frame limit further

Pick 256k as the limit.

While -Wno-frame-larger-than would make more sense for usage
in our test suite, the -Wno version seems to have no effect
if -Wframe-larger-than was already specified.

Use an (un)reasonably large value instead.

Fixes the build with clang:
../../tests/cputest.c:964:1: error: stack frame size of 33176 bytes
in function 'mymain' [-Werror,-Wframe-larger-than=]
mymain(void)
^
1 error generated.
Signed-off-by: NJán Tomko <jtomko@redhat.com>
Reviewed-by: NDaniel Henrique Barboza <danielhb413@gmail.com>

m4/virt-compile-warnings.m4

@@ -172,7 +172,7 @@ AC_DEFUN([LIBVIRT_COMPILE_WARNINGS],[
     # but using 1024 bytes sized buffers (mostly for virStrerror)
     # stops us from going down further
     wantwarn="$wantwarn -Wframe-larger-than=4096"
-    gl_WARN_ADD([-Wframe-larger-than=32768], [RELAXED_FRAME_LIMIT_CFLAGS])
+    gl_WARN_ADD([-Wframe-larger-than=262144], [RELAXED_FRAME_LIMIT_CFLAGS])
 
     # Extra special flags
     dnl -fstack-protector stuff passes gl_WARN_ADD with gcc