configure: use autobuilds to temporarily verify strict aliasing warnings.

Temporarily, When cross-compiling with gcc 3.0 or later, enable strict aliasing
rules and warnings. Given that cross-compiled targets autobuilds do not run the
test-suite, there is no risk of running code that violates strict aliasing rules

m4/cares-compilers.m4

@@ -931,6 +931,11 @@ AC_DEFUN([CARES_SET_COMPILER_WARNING_OPTS], [
             dnl Also, on gcc 4.0.X it is totally unbearable and complains all
             dnl over making it unusable for generic purposes. Let's not use it.
             tmp_CFLAGS="$tmp_CFLAGS"
+            dnl When cross-compiling with gcc 3.0 or later,
+            dnl enable strict aliasing rules and warnings.
+            if test "x$cross_compiling" = "xyes"; then
+              tmp_CFLAGS="$tmp_CFLAGS -fstrict-aliasing -Wstrict-aliasing=2"
+            fi
           fi
           #
           dnl Only gcc 3.3 or later