std_detect: Don't unnecessarily depend on libc for windows-msvc

[ChrisDenton]

I'm trying to reduce std's dependency on libc when it isn't actually needed.

Currently std_detect depends on libc for these features:

stdarch/crates/std_detect/Cargo.toml

Lines 37 to 39 in 9814fb0

	std_detect_file_io = [ "libc" ]
	std_detect_dlsym_getauxval = [ "libc" ]
	std_detect_env_override = [ "libc" ]

However, for windows-msvc, the only feature actually using libc in std_detect is std_detect_env_override

stdarch/crates/std_detect/src/detect/cache.rs

Lines 120 to 137 in 9814fb0

	if #[cfg(feature = "std_detect_env_override")] {
	#[inline]
	fn initialize(mut value: Initializer) -> Initializer {
	let env = unsafe {
	libc::getenv(b"RUST_STD_DETECT_UNSTABLE\0".as_ptr() as *const libc::c_char)
	};
	if !env.is_null() {
	let len = unsafe { libc::strlen(env) };
	let env = unsafe { core::slice::from_raw_parts(env as *const u8, len) };
	if let Ok(disable) = core::str::from_utf8(env) {
	for v in disable.split(" ") {
	let _ = super::Feature::from_str(v).map(|v| value.unset(v as u32));
	}
	}
	}
	do_initialize(value);
	value
	}

The easier way to address this issue is to make it so the on windows-msvc targets, libc is only used for this feature. The harder way is to use the Windows native equivalent of libc::getenv.

[dpaoliello]

@ChrisDenton getenv can be replaced with GetEnvironmentVariableA - would you be ok with a hand-coded binding, or would you want windows-rs pulled in so that the binding can be generated?

[ChrisDenton]

While this feature isn't enabled by our std builds, it could be, So a hand-coded binding would be better.

[dpaoliello]

While this feature isn't enabled by our std builds, it could be, So a hand-coded binding would be better.

Done: #1571