Well currently we pack bitfields with an algorithm that uses #ifdefs for GCC and MSVC builds. This feature tries to remove the hardcoded behavior and implement it as a runtime option. This should improve interop with other compilers. Currently I provided these for MSVC-style and GCC-style bitfield allocations. These, of course, can be extended with other strategies.

I am not sure that the fact that GCC has different types of bitfield allocations in different versions is a point against this feature. Consider that in our current code we don't use compiler bitfield allocation, we create the structure layout using our own algorithm, interop might be broken even if Python gets built with same version of GCC as the binary we want to interop with as long as algorithm is out of date. This patch should provide some flexibility in this matter.

Wouldn't a GCC44 constant provided at API level be better than saying "you can't interop with anything build with GCC 4.4 and up"? Or vice-versa, anything built with GCC < 4.4.