Unbreak PDF Import extension when building with modern boost. Similar
to the patch by amdmi3, link the extension with -lboost_system, but
only do this in the --with-system-boost case. Doing this unconditionally
would break building with the bundled boost because only the boost headers
are available and the boost libraries are not built. This is still a bit
pessimal because -lboost_system may be used when it is not strictly
necessary (when the system boost is old), but it is likely that this is
only relevant to FreeBSD and we are in the process of upgrading boost.
This fix should be acceptable upstream.

Re-enable the PDF Import extension by default.

When the PDF Import extension is enabled, promote boost from BUILD_DEPENDS
to LIB_DEPENDS.

Tested with PDFIMPORT both on and off. Also tested with PDFIMPORT on,
--with-system-boost disabled, and boost removed from *_DEPENDS.

PR: 207073, 199601