add SB-POSIX to ASD dependencies

[paulapatience]

Also guarded the SBCL dependencies behind the SBCL feature, as is done further below in the ASD file (though there remain various unguarded calls to SBCL packages throughout the source code), and removed the comment beside the SB-MPFR dependency, because it is used in more than just tests (in MPD-PI via MPD-RECIPROCAL).

[stylewarning]

I believe these dependencies can be guarded with specific ASDF syntax. Something like this.

[stylewarning]

(Side note: honestly, I should probably cordon off the TEST-PI into the test package where MPFR is expected. The main package isn't the right place I don't think. Just laziness on my part.)

[paulapatience]

I believe these dependencies can be guarded with specific ASDF syntax. Something like this.

I knew about :if-feature, but not about :feature for :depends-on. Thanks.

I've force-pushed the PR with :feature :sbcl instead of #+sbcl.

[stylewarning]

Thanks!