common: Add platform #ifdefs for non-cmake build convenience.

The cmake CMakeLists.txt file has logic for including or excluding certain
compilation units based on the platform: Windows, Linux or Mac OS X.
Other build systems are not as clever, it is convenient to also wrap
platform-specifics with #ifdef _WIN32 ... #endif to keep things simple on
the build side.

The STARTUPINFOA change is necessary for building apitrace
in unicode mode, on Windows.