configure: merge AC_CHECK_HEADERS calls
configure: merge AC_CHECK_HEADERS calls

Also removed some pointless AC_SUBST invocation, which would only be
necessary if the Makefiles were using the results of the header
detection, which they are not.

Signed-off-by: Max Horn <0706025b2bbcec1ed8d64822f4eccd96314938d0@quendi.de>

LordJZ/libflac,LordJZ/libflac,fredericgermain/flac,Distrotech/flac,waitman/flac,LordJZ/libflac,waitman/flac,fredericgermain/flac,Distrotech/flac,fredericgermain/flac,waitman/flac,fredericgermain/flac,waitman/flac,LordJZ/libflac,Distrotech/flac,Distrotech/flac,waitman/flac