filter SDKROOT for macos->macos build

[Dushistov]

fixes #530

The code mainly based on rustc source code,
but instead of removing it replace SDKROOT env variable,
because of it is important to find libc headers, like pthread.h

[alexcrichton]

Thanks for the PR! I would prefer to avoid modifying the environment of the calling process if possible, only updating the environment of spawned processes. Additionally this seems not super great to hardcode a path into this crate which may not be the same for all users? Would it be possible to somehow infer that? Are you sure that if the env var isn't set it won't work?

[Dushistov]

I would prefer to avoid modifying the environment of the calling process if possible, only updating the environment of spawned processes.

Done

Additionally this seems not super great to hardcode a path into this crate which may not be the same for all users? Would it be possible to somehow infer that?

Done

Are you sure that if the env var isn't set it won't work?

Yes, I used such script to find out what is really problem in #530:

export PATH=/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin:$PATH
#export SDKROOT=/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk
unset SDKROOT


cd /usr/local/var/cache//registry/src/github.com-1ecc6299db9ec823/libloading-0.5.2
"cc" "-O3" "-ffunction-sections" "-fdata-sections" "-fPIC" "-g" "-fno-omit-frame-pointer" "-m64" "-arch" "x86_64" "-Wall" "-Wextra" "-o" "/tmp/global_static.o" "-c" "src/os/unix/global_static.c"

Initially I inserted all environment variables into it and then binary search the problem env. variables.
As result only two environment variables matter: PATH and SDKROOT.
If I run this script it fails and prints pthread.h not found,
if I comment unset SDKROOT and uncomment SDKROOT all works fine.

So cc in /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin works only with setted SDKROOT, /usr/bin/cc works just fine without SDKROOT,
but I run all build under xcodebuild and it changes default cc and set SDKROOT unfortunately.

[alexcrichton]

Can this comment be clarified? The point of "removing is not enough" is only applicable when we're running clang directly out of the sysroot I suspect. Executing /usr/bin/clang doesn't require SDKROOT to be set, for example.

[Dushistov]

Done