PSA: "make check" and unused library dependencies

[Jack Bates <6n...@nottheoilrig.com>]

In #1408 [1], I added a script to the "make check" target that checks 
for programs that are linked with libraries they don't in fact use.
(It runs "ldd --unused --function-relocs" on all the programs that we 
install.)

To get this check to pass, I pruned the LIBS variables in our Makefiles, 
and added --as-needed to the linker flags.

I don't expect this check will fail on your platform (it passes on the 
platforms that I've tried), but if it does fail for you, I'd be 
interested to hear about it.

Thanks!

[1] https://github.com/apache/trafficserver/pull/1408