Fix broken conda(-forge) compilers on MacOS

I got stuck on this error for a long time:

ld: library not found for -lSystem

In the end, it turned out that the conda-build system was setting $CONDA_BUILD_SYSROOT to /opt/MacOSX10.10.sdk, which was a path that did not exist on my machine. I can't figure out where exactly the setting gets injected.

There may be a way to override it, but I thought it was easier just to make a symlink to that path. This command prints the sysroot for the main XCode install:

xcrun --show-sdk-path

Which may yield a result something like /Applications/XCode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.12.sdk.

See also conda-forge/conda-forge.github.io#824.