Permit make values to be indented or in env.

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 }.