beesh: use add_sourcesubdir to specify a source subdirectory

..to use when auto detecting buildtypes.

Sometime packages "hide" the sourcetree in a subdirectory you normaly have to
change to before executing the configure phase.

specify

add_sourcesubdir src

or

add_sourcesubdir cmake

in the bee file to enable this feature.

TODO: check how this relates to build_in_sourcedir
buildtypes: enable add_sourcesubdir for the cmake buildtype
Merge branch 'add_sourcesubdir'

* add_sourcesubdir:
  buildtypes: enable add_sourcesubdir for the cmake buildtype
  beesh: use add_sourcesubdir to specify a source subdirectory