[Feature request] Support $TARGET-pkg-config for cross-compilation

[jan-hudec]

My Debian system has following scripts in /usr/bin:

i686-linux-gnu-pkg-config
i686-w64-mingw32-pkg-config
pkg-config
x86_64-linux-gnu-pkg-config
x86_64-linux-gnux32-pkg-config
x86_64-pc-linux-gnu-pkg-config
x86_64-w64-mingw32-pkg-config

The prefixed scripts are symlinks to a wrapper that sets PKG_CONFIG_LIBDIR to the appropriate multilib directory to point to the correct package architecture. Since that is how libraries for cross-compilation toolchains are normally installed, it should work out of the box with them.

So I'd like to suggest to:

1. If $TARGET != $HOST, look for $TARGET-pkg-config and $GCC_TARGET-pkg-config script instead of pkg-config and
2. If it exists, the target_supported() function should return true and the script would be called.

The mapping from llvm-style $TARGET and gcc-style $GCC_TARGET is unfortunately not uniform, but looking at the list of targets both Rust and Debian know, two replacement rules should be sufficient:

1.-unknown- → - and
2. -pc-windows-gnu → -w64-mingw32

The scripts seem to be created when multi-arch is enabled for given target (with mingw/windows being a special case) and I only have that for amd64 (native), i386 and x32, but I believe these days it should work for qemu-emulated architectures too, so somebody can conceivably have arm-linux-gnueabi-pkg-config or aarch64-linux-gnu-pkg-config and if they do, it is going to work with the corresponding prefixed gcc linker.

(The curious x86_64-pc-linux-gnu-pkg-config is strangely a plain hardlink to pkg-config though the default gcc target is as expected just x86_64-linux-gnu with no -pc-; I believe it can be safely ignored)

[alexcrichton]

Sounds like a plausible idea to me!