win32.S: Add handling for position-independent code on Darwin

Newer versions of Darwin generate the necessary stub functions
automatically and just need a call instruction, but accomodating older
versions as well requires adding the stub.
Always use configure to detect whether global symbols need underscores

64-bit Windows already used this check; make it universal, and use it in
place of an ifdef on X86_WIN32, to handle non-Windows platforms that use
the underscore, such as Darwin.
Compile win32.S on 32-bit Darwin as well
Merge pull request #106 from joshtriplett/darwin-award

[3.1 blocker] Update OS X build system to include win32.S on 32-bit