META_MODE: Allow not ignoring host headers with NO_META_IGNORE_HOST_HEADERS.

See r301467 for more details on NO_META_IGNORE_HOST. Usually the full
list of host ignores should have no real impact on the host tools. The
headers however may reliably define what the ABI is for the host. It
may be useful to allow using the headers for the build but still not
caring about things like /bin/sh, /lib/libedit.so, etc.

Sponsored by: Dell EMC Isilon