configure.ac: don't rely on bashisms
93fb037d8409d9d46606c31d8a240e3963b72651 introduced unportable shell
syntax, which led to erros like
./configure: 3179: ./configure: CFLAGS+= -D__STDC_WANT_LIB_EXT1__=1: not found
in case a posix shell is used.
Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>