libFLAC.m4 : Prevent empty -L flag.
libFLAC.m4 : Prevent empty -L flag.

Fix libFLAC.m4 may set empty -L flag when configure is called without
arguments, provided of course that the target package's configure script
is properly re-generated against the fixed libFLAC.m4

Patch from Fabian Greffrath <fabian+debian@greffrath.com> via Debian.

waitman/flac,kode54/flac,Distrotech/flac,LordJZ/libflac,fredericgermain/flac,waitman/flac,kode54/flac,LordJZ/libflac,LordJZ/libflac,kode54/flac,Distrotech/flac,waitman/flac,waitman/flac,fredericgermain/flac,waitman/flac,kode54/flac,fredericgermain/flac,Distrotech/flac,fredericgermain/flac,Distrotech/flac,LordJZ/libflac