build.rs: Respect CXXSTDLIB

[jschwe]

mozjs/mozjs-sys/build.rs

Lines 572 to 574 in ad7c039

	if is_apple || is_freebsd || is_ohos {
	flags.push("-stdlib=libc++");
	}

We should first check CXXSTDLIB and respect it if set, and fallback to the hardcoded assumption only when not overriden.
cc-rs also respects this variable.