Clean up the following things:
1. Avoid the broken when use TARGETS in the command line, eg:
$ make -C tools/testing/selftests TARGETS=capabilities
make[1]: *** No rule to make target 'capabilities', needed by 'all'. Stop.
Replace TARGETS with BINARIES.
2. User need to provide cap-ng.h and libcap-ng.so for cross compiling.
Replace ':=' with '+=' for CFLAGS and introduce LDLIBS to archieve
it. Delete useless EXTRA_CLAGS at the same time.
3. Delete the duplicated definition which is already defined by
lib.mk.