Add MSVC support

Add MSVC support

Test for _WIN32 to check for building for Windows.
Test __MINGW32__ to check for using MinGW compiler (on Windows).
If _WIN32 is defined and __MINGW32__ is not defined, assume Microsoft compiler, headers and libraries.
There are other ways this could be done, testing _MSC_VER for example, but these tests work.
Tested with Visual Studio 2005, MinGW 4.6.1 on Windows (mingw-get-inst-20110802.exe) and gcc 4.6.1
in Ubuntu 11.10 (Ubuntu/Linaro 4.6.1-9ubuntu3).