No mention of sigemptyset seen in source code; dropped patch

Signed-off-by: Ian Leonard <antonlacon@gmail.com>

This updates flac to the new upstream release v1.3.1 to fix two known CVEs.

Additionally the commit changes the following:
 * Drop custom Build/Configure recipe in favor to CONFIGURE_ARGS
 * Refreshes patches and removes changes to *.in files since they're
   regenerated by autoreconf anyway
 * Remove now obsolete 020-libFLAC-remove-altivec-options.patch
 * Add an md5sum for the source archive
 * Passes --enable-static to configure to force producing a libflac.a

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>