0.17

This releases adds support for gcc 9 (along with continued support for gcc 4.6, 4.7, 4.8, 4.9, 5, 6, 7, and 8).

Unfortunately, the reference-count checker no longer works for gcc 7 onwards, so it is disabled when embedded in those builds of gcc.

Additionally, this release contains the following improvements:

• the plugin can now be built in a separate build directory from the source directory (thanks to Tom de Vries)
• gcc-with-cpychecker gained a --cpychecker-verbose option