Add option to include shared library in wheel

Before, the RPATH was always set to the build directory, which is neither good for building a locally installable wheel nor for deb or rpm packaging.

Now, if the SEEDEE_WHEEL_INCLUDE_LIB option is enabled, the seedee library is included in the wheel and the RUNPATH of the extension library is set accordingly. If it is disabled, the RUNPATH is not set at all.

Additionally, a builtin setuptool option is used to set the RUNPATH instead of adding the compiler flags directly.