Rebuild build/link system to avoid RTLD_GLOBAL

[mikeboers]

Currently, LibTomCrypt is compiled into tomcrypt/_core.so. In order for those functions to be accessible to the other Cython modules, we force the .so to be loaded in RTLD_GLOBAL mode.

Unfortunately, this pollutes the global namespace.

Potential options are:

• compile LibTomCrypt into its own _tomcrypt.so, which is still loaded with RTLD_GLOBAL, hoping to polute less;
• rebuild everything using ctypes;
• merge everything into a single tomcrypt.so which is all Cython;
• wrap every C function in a Cython function so it can be exported via the Cython capi;
• hack every C function into the Cython capi export table.

[mikeboers]

Try redefining mode as RTLD_GLOBAL after import:

import ctypes
from . import _core
ctypes.PyDLL(_core.__file__, mode=ctypes.RTLD_GLOBAL)