robotpkg

Hi,

While trying to compile some ROS packages on Arch with python3.7, I had to do those changes:
https://github.com/nim65s/robotpkg/commit/11589e35b543392a7ba9b3c00f4642dc0fb7c956
https://github.com/nim65s/robotpkg/commit/ac874aa36d97ae2a1aa43001b67dffa2b497adfb

Also, with a recent CMake, I had to apply
https://github.com/nim65s/robotpkg/commit/0bffd8c0c92ca924abb2892716d453497485f3f7
Which comes from https://github.com/ros/catkin/pull/975 but it might be fixed upstream soon.

I had an issue with PYTHON (and therefore PYTHON_SOABI) not beeing defined in a package depending on py-netifaces I wanted to build, so I manually added a `SYSTEM_FILES.python37 = /usr/bin/python3.7` in a `.sysdepends` file, but I must have been missing something.