Include -isysroot linker arg on macos

[frgtn]

Adds -isysroot linker flag when building extensions on darwin target.
Uses xcrun --show-sdk-path to determine isysroot value.

This is a minimal patch that allows building lxml on MacOS Sierra 10.12.x with XCode 9.

Based on suggestions found in https://forums.developer.apple.com/thread/87829
and addresses https://bugs.launchpad.net/lxml/+bug/1727864

[scoder]

Thanks!
What happens if "xcrun" is not installed?
I just want to make sure it doesn't break any other setup.py commands or something like that.

[frgtn]

As far as I know xcrun is only available on MacOS when XCode (along with command line utilities) is installed. XCode is needed to build C extensions so if users are able to build extensions they should already have xcrun installed.

Here is a man page for xcrun: https://developer.apple.com/legacy/library/documentation/Darwin/Reference/ManPages/man1/xcrun.1.html

The xcrun command will only be invoked in MacOS (`sys.platform == 'darwin') so it shouldn't cause problems for other platforms.

[kespindler]

I just encountered this issue on my mac, and this PR is what fixed it. Specifically, I had to do:

pip3 install cython
pip3 install git+git://github.com/frgtn/lxml.git@5bfc9f03ffb6458ed579f4166cc9825ed7d46315

Really appreciate the PR!

[kespindler]

👍