Enable automake's subdif-objects option.

automake 2.0 will enable subdir-objects by default.
1.14 now issues warnings when sources are used from subdirectories, yet subdir-objects option is not used.
libnih compiles with and without this option. with this option enabled, no warnings are displayed for each
test_foo_SOURCES = tests/test_foo.c
and the test_foo.o is placed in tests/test_foo.o instead of ./test_foo.o