unix mem_info(): Dump GC info only if it's enabled.
nlrx86.S: Another ifdef for win32 symbol underscoring issues.
py: Add win32-specific header for alloca().
unix file: mingw32 has STDIN_FILENO and friends in stdio.h .

TODO: Never "optimize" includes any more!
unix windows: mingw32 doesn't have realpath(), use _fullpath() instead.
windows: Unbreak mingw32 build (cross-compiling under Linux).