Fix libdns CFLAGS
Context:
What happens here is that on FreeBSD, building with --enable-dnstap
works, but breaks with --enable-dnstap --without-lmdb. This is
because both fstrm and LMDB are installed into /usr/local, but
checking for LMDB inadvertently updates CFLAGS with LMDB-provided
CFLAGS due to a bug in m4/ax_lib_lmdb.m4 (which is also fixed by
this MR). When LMDB support is disabled, -I/usr/local/include no
longer appears in CFLAGS and thus a dnstap-enabled build fails.