Fix stdcall ABI call convention.

FFI_STDCALL is no define, so stdcall was never used.
This fixes issue #302 and adds a test case for stdcall callbacks..
Fix compuation of bytes stored on the stack for stdcalls, used for function decoration.
Fix return type of callback function to rb_thread_call_with_gvl().

The return type is not used but caused a compiler warning:
  Function.c: In function 'callback_invoke':
  Function.c:479:9: warning: passing argument 1 of 'rb_thread_call_with_gvl' from incompatible pointer type [enabled by default]
  Function.c:102:14: note: expected 'void * (*)(void *)' but argument is of type 'VALUE (*)(void *)'

This addresses issue #297.
Merge pull request #309 from larskanis/fix-return-of-callback_with_gvl

Fix return type of callback function to rb_thread_call_with_gvl().
update/correct benchmarks
Merge pull request #348 from tduehr/benchmarks

update/correct benchmarks
Remove compatibily code for older rake-compiler versions.
It was only useful as long 0.9.0 wasn't relesed.
Fix typo in checking RTLD defines.
Merge pull request #349 from larskanis/fix_typo_RTLD_NOW

Fix typo in checking RTLD defines.
Don't include NetBSD version in ARCH. NetBSD is BSD.
Add missing intptr_t and uintptr_t (sync with FreeBSD and OpenBSD).
fix tests for netbsd
Merge pull request #351 from tduehr/netbsd

Netbsd
Merge pull request #298 from larskanis/mingw-support

Bump rake-compilers version to 0.9.2 to ensure x64-mingw32 is built correctly.
Merge pull request #306 from larskanis/fix-stdcall

Fix stdcall ABI call convention.
fix ld scripts in freebsd
Merge pull request #328 from tduehr/freebsd_fix

Fix library loading on FreeBSD systems
Merge pull request #307 from larskanis/fix-stdcall-decoration

Fix compuation of bytes stored on the stack for :stdcall, used for function decoration.
fix gemspec typo