Move writing of config module to library routine
Multi-arch support in writing config mode file
Write pkgconfig files as well as CMake

Note that using pkg-config is NOT recommended because of:

1. It doesn't know which language you are using, so language-specific
   options such as -std=c++11 or -std=c99 cannot be passed along. Yet,
   the build will corrupt if you don't get these correct.

2. It has no means of communicating which defines that should be set
   in config.h, effectively disabling them all, possibly leading to
   inconsistencies between the library and the executable.