configure.ac: fix bashism (#803) #32

Job	Run time
build (32-bit-words, --disable-64-bit-words)	4m 16s
build (integer-only-library, -DFLAC__INTEGER_ONLY_LIBRARY)	3m 44s
build (no-asm, --disable-asm-optimizations)	2m 36s
build (no-ogg, --disable-ogg)	2m 36s
	13m 12s

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