Bug 939039 - Make FINAL_LIBRARY imply FORCE_STATIC_LIB. r=gps

This also moves the LIBXUL_LIBRARY implies FORCE_STATIC_LIB test in the emitter
instead of config.mk, since both variables are defined in moz.build exclusively.
backend.mk can end up with both LIBXUL_LIBRARY and FORCE_STATIC_LIB defined, but
it's just fine.
Bug 939039 - Remove now useless FORCE_STATIC_LIB definitions. r=gps
Bug 939042 - Remove library building related variables in directories that build no C/C++ sources. r=mshal
Bug 939044 - Remove the last uses of the MODULE variable in makefiles. r=gps
Bug 939044 - Remove most definitions of MODULE. r=mshal
Bug 939044 - Rename remaining MODULE definitions to XPIDL_MODULE. r=mshal
Bug 939044 - Make the build frontend and backend forget about MODULE. r=gps
Bug 914245 - Move FORCE_SHARED_LIB to moz.build. r=mshal
Bug 939071 - Kill SHORT_LIBNAME. r=ted