CMake 3.0.12 uses SWIG_DIR to set SWIG_LIB, used as the library
install location, which defaults to
$(STAGING_HOSTPKG)/share/sig/(SWIGVERSION).  If it is set, then the
original installed swig library directory is ignored, and compilation
fails:
[  0%] Swig source
 Error: Unable to find 'swig.swg'
:3: Error: Unable to find 'python.swg'

Instead of setting it manually, let the default be used, which works
well.

Also, cmake picks up python3, while package wants 2.7, so we add
-DPython_ADDITIONAL_VERSIONS=2.7 to force use of 2.7, not 3.x.

Signed-off-by: Eneas U de Queiroz <cote2004-github@yahoo.com>