Encoded CFLAGS for supporting spaces

[MarijnS95]

According to https://github.com/rust-lang/cc-rs/#external-configuration-via-environment-variables spaces nor escapes for spaces are not supported in e.g. C(XX)FLAGS. In xbuild we use CFLAGS to set a --sysroot path, and this can occasionally contain spaces on Windows machines (because the toolchain is unpacked to a user folder, and users like to have Firstname Surname as username and profile directory).

A likable solution is new CARGO_ENCODED_RUSTFLAGS-like variables, where spaces within individual args are supported by requiring the user to replace space delimiters in between separate arguments with the \x1f ASCII Unit Separator: https://doc.rust-lang.org/cargo/reference/environment-variables.html, to allow more mechanical piecing-together of these variables without messing around with nested quotes or backslash-escapes.

Referencing rust-mobile/xbuild#124, where this is one of the deciding factors to build our Android compiler support differently.

[joshtriplett]

While that might be possible to address in cc-rs, it still seems likely to break in other build systems, such as invocations of make, which don't have a solution to this problem.

[MarijnS95]

@joshtriplett does cc forward to other build systems or only expand these flags into argv before invoking the compiler (which should be possible and support spaces by splitting on the separator char)? Or is this to maintain feature parity with other build systems outside of cc-rs that also process CFLAGS?

[joshtriplett]

I'm talking about the latter: cc-rs doesn't seem like the right place to unilaterally propagate a new standard for passing CFLAGS that no other build system will understand.

[thomcc]

There admittedly are plenty of people just using cc so it's not entirely out of the question. I have mixed feelings about it though, for sure.