Fix build without Python support.

Only search for BoostPython3 if Python was found. Otherwise,
${PYTHONLIBS_VERSION_STRING} expands to nothing and the
find_boost_python3 macro gets called with invalid arguments.

CMakeLists.txt

@@ -45,6 +45,7 @@ macro_log_feature(
     "Python 3 is used for some Calamares job modules."
 )
 
+if ( PYTHONLIBS_FOUND )
     include( BoostPython3 )
     find_boost_python3( 1.54.0 ${PYTHONLIBS_VERSION_STRING} CALAMARES_BOOST_PYTHON3_FOUND )
     macro_log_feature(
@@ -55,6 +56,7 @@ macro_log_feature(
         FALSE "1.54.0"
         "Boost.Python is used for interfacing with Calamares job modules written in Python 3."
     )
+endif()
 
 if ( PYTHONLIBS_NOTFOUND OR NOT CALAMARES_BOOST_PYTHON3_FOUND )
     set( WITH_PYTHON OFF )