Bail configuration on missing package

If we're configured to use pkg-config (or pcre-config) and the tool is
not available or does not know about the package we ask for, that should
be a fatal configuration error.

We should not silently ignore the missing package, then try to compile,
and have missing header warnings from the compiler.  Eg, if we're told
to support GSASL, we'll try to compile the client code, and without
compiler flags, we'll either fail to compile (missing headers) or fail
to link, which obscures the source of the errors.

This change will only break people who had builds set to have Exim
depend upon non-existent packages, and that _needs_ to break.

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