metaflac: print error if no operations #413

Job	Run time
build (no-asm, --disable-asm-optimizations)	2m 28s
build (integer-only-library, -DFLAC__INTEGER_ONLY_LIBRARY)	2m 23s
build (no-ogg, --disable-ogg)	2m 36s
build (32-bit-words, --disable-64-bit-words)	2m 28s
	9m 55s

Provide feedback