compile_extension_module demo fails when run via CMake

[Johan Hake]

When:

make -j X run{_regressiontest,tests}

are run with X > 1 compile_extension_module demo fails due to some failed dependency for the SWIG module and the passed source file. One could maybe compile source code independently in the generated CMakeList.txt file?

[Johan Hake]

Should be fixed in instant commit 7abf4e7f734c433e835c3e0054e17faadc1d913d