gcc 3.4:
* The C, C++, and Objective-C compilers can now handle source files
written in any character encoding supported by the host C library. The
default input character set is taken from the current locale, and may be
overridden with the -finput-charset command line option. In the future
we will add support for inline encoding markers.

MinGWのランタイムとかCygwinって日本語ロケールはサポートしてるのかな?