buildenv/win32-static: treat libsndfile.a as a MinGW library in combine-libsndfile-libs.bash.
buildenv/win32-static: add new UTF8 libraries to the libFLAC lib.

This change adds build instructions for utf8_static.lib
and win_utf8_io.lib (both new in 1.3.0) to build-libflac.bash.

Also, instead of simply copying libFLAC_static.lib to libFLAC.a,
it invokes lib.exe to compose a libFLAC.a that consists of all of
the required libraries: libFLAC_static.lib, utf8_static.lib and
win_utf8_io.lib.

This change also updates combine-libsndfile-libs.bash to
be able to extract symbols from the new utf8_static.lib and
win_utf8_io.lib static libraries. Their object files live
in 'Release/<filename>.obj' inside the archive, which
the script didn't previously consider.