Add GENERATE_SWIG_WRAPPERS option to control the generation of swig wrappers independently of whether we're building from git

The swig wrappers don't really depend on git (but rather on swig) and there can be
situations the builder wants to generate the wrappers also from a tar ball.

po/CMakeLists.txt

@@ -179,7 +179,7 @@ IF(BUILDING_FROM_VCS)
   )
   add_custom_target (pot DEPENDS gnucash.pot)
 ENDIF()
-dist_add_generated(gnucash.pot)
+dist_add_generated (${BUILDING_FROM_VCS} gnucash.pot)
 
 ADD_CUSTOM_TARGET(check-po
         COMMAND ${CMAKE_COMMAND}