For a source:
|cdef extern from "<foo>":
|    cdef void foo_cpp() nogil except +
|
|cdef call_foo() nogil:
|    foo_cpp()

We used to generate something like this to actually call foo_cpp() in call_foo():
|try{foo_cpp();}
|catch(...) {
|  Py_BLOCK_THREADS; __Pyx_CppExn2PyErr(); Py_UNBLOCK_THREADS
|  `code.error_goto(self.pos))`
|}

where Py_BLOCK_THREADS expands to "PyEval_RestoreThread(_save);".
__Pyx_CppExn2PyErr() (and alternatives, see SimpleCallNode) calls CPython A API
methods so it needs to be guarded in a nogil environment.

One problem is that "PyThreadState *_save" is only declared by "with nogil:"
block, so a .cpp file that doesn't compile is generated for the above code.

However, I think the real issue is that Py_(UN)BLOCK_THREADS is inappropriate
here, as it isn't allowed to be called recursively and is valid only directly
in a Py_BEGIN_ALLOW_THREADS ... Py_END_ALLOW_THREADS. IMO PyGILState_Ensure()
and PyGILState_Release() (through `code.put_ensure_gil()` and a friend) is the
correct thing to call here as it is allowed to be called recursively and
actually ensures the current thread can call CPython C API.

This patch does exactly this (and it breaks the generated code to multiple
lines as it would be way too long otherwise), plus it extends the
cpp_exceptions_nogil.pyx test with above example that doesn't compile with this
fix not applied.

Note that we explicitly pass declare_gilstate=True to put_ensure_gil(), as
PyGILState_Ensure() docs state that recursive calls to it must not share the
PyGILState_STATE. C++-style declaring a variable inside a block should be
no-problem here, as try{} .. catch() is obviously valid only in a C++ code.

fix indentation error in userguide

move exception class into shadow function as we may not want to export it under the cython.* namespace at this point

removed broken minivect related code from branch since minivect will not be released as part of 0.18

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.

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

--HG--
extra : transplant_source : 9%EA%CC%A6%3D%1B9R%EF%0DmM%CFZ%18%F3%EC%06%3B%B7

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

--HG--
extra : transplant_source : %A8%F23%AF%26%BC%82y1%86S%1Ac%D3%40%089o%DCQ

--HG--
extra : transplant_source : %E6%CA%F8%11%E8%81u%B9%95%3D%27%C1%0F%F3O%8A%12%3Cnl

--HG--
extra : transplant_source : U%15%0B%8E%81%02%F2kE%AA%07u%EF%82%3D14%F1C%86

--HG--
extra : transplant_source : H%91%CF%08t%B1%908%AE%26%81%1B%F9%2C%9A%3Fh%ECWK

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