bpo-28552: Fix distutils.sysconfig for empty sys.executable (GH-12875)

bpo-28552: Fix distutils.sysconfig for empty sys.executable (GH-12875)

bpo-28552, bpo-7774: Fix distutils.sysconfig if sys.executable is
None or an empty string: use os.getcwd() to initialize project_base.

Fix also the distutils build command: don't use sys.executable if
it's evaluated as false (None or empty string).