configure.ac: fix bashism (#803) #25

Job	Run time
build (32-bit-words, --disable-64-bit-words)	2m 31s
build (integer-only-library, -DFLAC__INTEGER_ONLY_LIBRARY)	2m 40s
build (no-asm, --disable-asm-optimizations)	2m 24s
build (no-ogg, --disable-ogg)	2m 18s
	9m 53s

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