Changes

configure.ac: use subdir-objects

Fix a bunch of warnings:
  src/Makefile.am:1: warning: source file 'gen/TCAP_DialoguePDU.c' is in a subdirectory,
  src/Makefile.am:1: but option 'subdir-objects' is disabled
  src/Makefile.am:1: warning: source file 'gen/TCAP_AUDT-apdu.c' is in a subdirectory,
  src/Makefile.am:1: but option 'subdir-objects' is disabled
  src/Makefile.am:1: warning: source file 'gen/NULL_ber.c' is in a subdirectory,
  src/Makefile.am:1: but option 'subdir-objects' is disabled
  src/Makefile.am:1: warning: source file 'gen/asn_codecs_prim_ber.c' is in a subdirectory,
  src/Makefile.am:1: but option 'subdir-objects' is disabled
  …

Change-Id: I800c8d8e39923a8dd17e3bf1e5aeb1cf691383d8