It appears some make(1)s are not complaining about variables defined
with leading whitespace on the line. Permit that where we can, for the
lookups, but it's not tenable for CFLAGS_DYNAMIC.
Some people are specifying knobs on the make command-line, so we get
them via the environment.
Tested: indented LOOKUP_CDB and commented out LOOKUP_DNSDB, supplying it
via { make LOOKUP_DNSDB=yes }. { exim -d --version } shows both are
built-in, no results from { fgrep DNSDB build-*/Makefile }.