Reset GNUMAKEFLAGS after parsing.

If we don't do this we'll continually add flags on recursion.  This
is mainly for users to set in their environment before invoking make.
Portability for Guile 1.8.  Force UTF-8 encoding.
Fix build failures on EMX for output-sync support.
Solaris /bin/sh can't handle multiple options ("-e -c").
Fixes for building with MSVC.
Final fixes for obscure output-sync errors.
Fix portability issues for Guile 1.8.