cmake: Allow to build with system libraries and other improvements

[cryptomilk]

This patchset allows you to build piper-phonemize with system libraries instead of fetching sources from the internet. For this a new option has been introduced BUILD_EXTERNAL which will enable building with external sources. The default in cmake is OFF in the Makefile it is ON.

This also generates a cmake config file so you can consume the library using find_package(piper_phonemize) from other cmake files. It also fixes some issues if you use it as an external project.

You can see it building for openSUSE against system libraries here:

https://build.opensuse.org/package/show/home:gladiac/piper-phonemize