Added macOS SDK install path to library and include directories

[radarhere]

Resolves #4973 by using (as suggested) xcrun --show-sdk-path to add the SDK install path for library and include directories.

This same solution was also very popular in https://github.com/python-pillow/Pillow/issues/3438#issuecomment-575161585

[hugovk]

Confirmed the 7.2.0 tag doesn't build (macOS Mojave):

Details

$ pip install -e .
Obtaining file:///Users/hugo/github/Pillow
Installing collected packages: Pillow
  Running setup.py develop for Pillow
    ERROR: Command errored out with exit status 1:
     command: /Library/Frameworks/Python.framework/Versions/3.9/bin/python3.9 -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'/Users/hugo/github/Pillow/setup.py'"'"'; __file__='"'"'/Users/hugo/github/Pillow/setup.py'"'"';f=getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' develop --no-deps
         cwd: /Users/hugo/github/Pillow/
    Complete output (58 lines):
    running develop
    running egg_info
    writing src/Pillow.egg-info/PKG-INFO
    writing dependency_links to src/Pillow.egg-info/dependency_links.txt
    writing top-level names to src/Pillow.egg-info/top_level.txt
    reading manifest template 'MANIFEST.in'
    warning: no files found matching '*.c'
    warning: no files found matching '*.h'
    writing manifest file 'src/Pillow.egg-info/SOURCES.txt'
    running build_ext


    The headers or library files could not be found for zlib,
    a required dependency when compiling Pillow from source.

    Please see the install instructions at:
       https://pillow.readthedocs.io/en/latest/installation.html

    Traceback (most recent call last):
      File "/Users/hugo/github/Pillow/setup.py", line 864, in <module>
        setup(
      File "/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/site-packages/setuptools/__init__.py", line 153, in setup
        return distutils.core.setup(**attrs)
      File "/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/distutils/core.py", line 148, in setup
        dist.run_commands()
      File "/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/distutils/dist.py", line 966, in run_commands
        self.run_command(cmd)
      File "/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/distutils/dist.py", line 985, in run_command
        cmd_obj.run()
      File "/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/site-packages/setuptools/command/develop.py", line 34, in run
        self.install_for_development()
      File "/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/site-packages/setuptools/command/develop.py", line 136, in install_for_development
        self.run_command('build_ext')
      File "/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/distutils/cmd.py", line 313, in run_command
        self.distribution.run_command(command)
      File "/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/distutils/dist.py", line 985, in run_command
        cmd_obj.run()
      File "/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/distutils/command/build_ext.py", line 340, in run
        self.build_extensions()
      File "/Users/hugo/github/Pillow/setup.py", line 694, in build_extensions
        raise RequiredDependencyException(f)
    __main__.RequiredDependencyException: zlib

    During handling of the above exception, another exception occurred:

    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "/Users/hugo/github/Pillow/setup.py", line 918, in <module>
        raise RequiredDependencyException(msg)
    __main__.RequiredDependencyException:

    The headers or library files could not be found for zlib,
    a required dependency when compiling Pillow from source.

    Please see the install instructions at:
       https://pillow.readthedocs.io/en/latest/installation.html


    ----------------------------------------
ERROR: Command errored out with exit status 1: /Library/Frameworks/Python.framework/Versions/3.9/bin/python3.9 -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'/Users/hugo/github/Pillow/setup.py'"'"'; __file__='"'"'/Users/hugo/github/Pillow/setup.py'"'"';f=getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' develop --no-deps Check the logs for full command output.

And this branch does:

Details

$ pip install -e .
Obtaining file:///Users/hugo/github/Pillow
Installing collected packages: Pillow
  Running setup.py develop for Pillow
Successfully installed Pillow