Disable system site-packages via GENTOO_CPYTHON_BUILD var

Do not add system site-packages directory to sys.path
if GENTOO_CPYTHON_BUILD variable is set.  This solves multiple issues
while building and testing CPython, particularly test suite issues from
installed .pth files, Python modules (particularly docutils) and sandbox
violations from attempting to write byte-compiled modules back.

Signed-off-by: Michał Górny <mgorny@gentoo.org>

Author: mgorny

Lib/site.py

@@ -323,6 +323,7 @@ def addsitepackages(self, prefixes=None):
         global PREFIXES when *prefixes* is None.  Each directory's startup
         data is accumulated for later processing by process().
         """
+        if os.environ.get("GENTOO_CPYTHON_BUILD") and prefixes is None: return
         _trace("Processing global site-packages")
         for sitedir in getsitepackages(prefixes):
             if os.path.isdir(sitedir):