To allow modules to be separatebly buildable, you need to follow these rules:

in configure.ac:

1) declare target in proper format, eg.:
        AC_SUBST(SCREEN_CAP_LIB_TARGET, "lib/ultragrid/vidcap_screen_cap.so.$video_capture_abi_version")
2) give the target to LIB_TARGETS
3) propagate variables you need (typically *_INC, *_LIB and *_OBJ
2) give objects to LIB_OBJS

in Makefile.in
1) create declared target - this will be separate library, so make sure that every dependency is tracked within that librarz

