probe for broken restrict

[rurban]

with -O2 and inlining or loop-unrolling restrict is broken since gcc 5 and clang 6 until now.

in clang bug added with llvm r305938
rust-lang/rust#54878
https://bugs.llvm.org/show_bug.cgi?id=39282
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87609

Add a m4 probe and undefine it then.

[rurban]

Something like this:

AC_DEFUN([AX_RESTRICT],
[
AC_CACHE_CHECK([for restrict usability], [ax_cv_restrict_usable], [
AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[
void copy(int * restrict to, int * restrict from) {
	*to = *from;
}
void test(int *a, int *b) {
	for (int i = 0; i < 4; i++) {
		copy(&b[i & 1], &a[i & 1]);
	}
}
int main() {
	int ary[] = {0, 1, 2};
	test(&ary[1], &ary[0]);
	#printf("%d %d %d\n", ary[0], ary[1], ary[2]);
	return ary[0] == 2;
}
# 2 2 2 is correct
# 1 2 2 wrong
]], [[ax_cv_restrict_usable=restrict]], [[ax_cv_restrict_usable=]])])
])
    if test -z $ax_cv_restrict_usable; then
      AC_MSG_RESULT([no])
      AC_DEFINE_UNQUOTED(restrict, $ax_cv_restrict_usable, [If restrict is usable with this compiler])
    else
      AC_MSG_RESULT([yes])
    fi
]) dnl AX_RESTRICT

[rurban]

For now I disabled it, as I don't trust the restrict check (e.g. gcc-8 passes), but removed restrict from two suspicious candidate functions in decode_r2007. copy_bytes_2, copy_bytes_3.