tune/speed is up to date

[Seth Troisi]

I'm coming from Bazel and don't have a lot of experience with Make so
please excuse me if there's a much simpler answer I'm just not seeing.

tune/speed seems to be statically linked, this makes sense, BUT it makes
testing a little harder because this no longer works.

$ vim mpz/nextprimes.c
$ make
$ make -C tune/ speed
make: 'tune/speed' is up to date.
$ ./tune/speed ...
(RUNS WITHOUT NEWLY CHANGED CODE)

My work flow is to instead clean and rebuild clean each time

<change mpz/nextprimes.c>
$ vim mpz/nextprimes.c
$ make
$ make -C tune/ clean speed -j4
or more recently
$ rm tune/speed; make -C tune/ speed -j4
$ ./tune/speed ...

Because gmp is well written (IMHO) rebuilding tune/speed only takes
14seconds, but it's not ideal.

I "solved" this by adding `.PHONY: $(EXTRA_PROGRAMS)`
to tune/Makefile.am and wanted to checking if that should be upstreamed, or
if there's another way to do this that I'm just unaware of.

Thanks,
Seth