Fix build on FreeBSD
- tidy up CMake bits
- simplify build with IMPORTED targets; this is actually the important bit, since it's much more convenient to write ARPA2::Quick-DER than to expand it by-hand in the CMakeLists.txt; also handles include-directories transparently. This needed more shuffling than I anticipated, because apxs is really picky about where
-I
flags can be specified on its command-line.