unix: define python_stdlib_platform_config PYTHON.json field

This will help consumers find the path containing files used to
build embedded binaries. It appears Windows does not populate an
equivalent directory.

Closes #48.

Author: indygreg

cpython-unix/build-cpython.sh

@@ -400,6 +400,7 @@ metadata = {
     "python_paths": {},
     "python_exe": "install/bin/python%s%s" % (sysconfig.get_python_version(), sys.abiflags),
     "python_major_minor_version": sysconfig.get_python_version(),
+    "python_stdlib_platform_config": sysconfig.get_config_var("LIBPL").lstrip("/"),
 }
 
 root = os.environ["ROOT"]

docs/distributions.rst

@@ -154,6 +154,17 @@ python_stdlib
 
    (Deprecated in version 5 in favor of ``python_paths``.)
 
+python_stdlib_platform_config
+   Relative path to a ``config-<platform>`` directory in the standard
+   library containing files used to embed Python in a binary.
+
+   This is a standard directory present in POSIX Python installations
+   and is not specific to this project.
+
+   The key may be absent if no platform config directory exists.
+
+   (Version 5 or above only.)
+
 python_stdlib_test_packages
    Array of strings of Python packages that define tests. (Version 4 or above
    only.)