lib: Make pcre, libmagic and hivex libraries optional.

lib: Make pcre, libmagic and hivex libraries optional.

This change makes these libraries optional.  If they are not
available at compile time then certain core API features will
be disabled (see below).

This also changes PCRE detection to use pkg-config instead
of the ad hoc autoconf checks.

The large inspect.c file has been split out into separate
function-specific files.

  file-architecture: requires pcre & libmagic
  inspection: requires pcre & hivex