build: Use Automake 'subdir-objects' option
* gettext-runtime/tests/Makefile.am (AUTOMAKE_OPTIONS): Add 'subdir-objects'.
* gettext-tools/libgettextpo/Makefile.am (AUTOMAKE_OPTIONS): Likewise.
* gettext-tools/libgrep/Makefile.am (AUTOMAKE_OPTIONS): Likewise.
* gettext-tools/src/Makefile.am (AUTOMAKE_OPTIONS): Likewise.
