Fix a build error on MinGW systems.

Some versions of the MinGW system headers rely on the types off_t and off64_t
being defined, but they will not be defined if -ansi is enabled.
(This seems to affect MinGW systems using GCC 4.8; I haven't checked other
GCC versions yet.)

configure.ac:
scripts/mgnuc.in:
	Do not compile with -ansi on MinGW systems.