Makefile.am: Don't use non-portable [[ syntax
POSIX shell doesn't support [[ syntax, so using it can causes command not found errors.
This avoid showing /bin/sh: [[: not found
when building on FreeBSD.
POSIX shell doesn't support [[ syntax, so using it can causes command not found errors.
This avoid showing /bin/sh: [[: not found
when building on FreeBSD.