configure.ac : Use '-static-libgcc' with mingw-w64-* compilers.

Without this, libsndfile-1.dll depends on libgcc_s_sjlj-1.dll which
is a pain in the neck. Tried a number of things to fix this, but in
the end it was a suggestion from Jean-Baptiste Kempf on the
mingw-w64-public mailing list that solved it.

Suggested-by: Jean-Baptiste Kempf <jb@videolan.org>