Fix compilation with MinGW. Right now, compilation out of the box with latest MinGW is broken due to bugs in header files of mingwrt 4.0.3 in strict ANSI mode, cf. https://sourceforge.net/p/mingw/bugs/2024/ https://sourceforge.net/p/mingw/bugs/2046/ * builds/unix/configure.raw: Don't set `-ansi' flag for MinGW.