Do not pass -ansi to GCC on any system.

Passing -ansi to GCC restricts us to the C90 dialect of C and prevents us from
using C99 library features with GCC.  Since every version of GCC we support
provides the relevant C99 features there is not much point to imposing this
restriction.  As already noted in the configure script the use of -ansi was
also causing problems with the header files on MinGW systems.  Finally, I do
think it is reasonable to require users to use the C90 dialect of C in
foreign_proc and foreign_code pragma bodies by default.

NOTE: this is *not* an invitation to start using C99 language features
throughout the runtime and library -- at this point, we (unfortunately) still
maintain compatibility with older versions of MSVC that don't support these
features.

configure.ac:
	Do not pass -ansi to GCC.  Add an explanation of why.

scripts/mgnuc.in:
	Update the description of the what the --no-ansi option does.
	(XXX there's _a lot_ of out-of-date stuff in this script.)