fix import of pyx modules when '' is in sys.path

If '' is in sys.path and a module is found the package_path
is relative and breaks the build process.

Use OS-dependent directory separator - a / on windows is interpreted by ...

Use OS-dependent directory separator - a / on windows is interpreted by LINK as a command line switch

BF: replace show-ref with rev-parse --verify for determining current treeish location

--HG--
extra : rebase_source : 5e91ffb0a20724dc92084bb68288cb6000814dca

generate safer cleanup code that also works in the case something goes badly wrong early at module init time

--HG--
extra : rebase_source : 015495e6aee6c59b11ee9169d2ad597ff54bfd0c

Conflicts:
	CHANGES.rst
	Cython/__init__.py