Fix Mingw build.

Following changes are done.

Add missing headers.
Replace _snprintf with snprintf. It is already changed to _snprintf for MSVC.
Add a file in the build for autoconf.
Call DynamicLoaderWindows::Terminate and DynamicLoaderWindows::Initialize only for MSVC build.

Reviewed in http://reviews.llvm.org/D7536.

llvm-svn: 228822