zyre/zyredir does not build - defconstant redefinition

[quicklisp]

I get this: Unhandled SB-EXT:DEFCONSTANT-UNEQL in thread #<SB-THREAD:THREAD "main thread" RUNNING {10005604C3}>: The constant +ZYREDIR+ is being redefined (from #P"/tmp/zyre/" to #P"/tmp/zyre/")

Pathnames aren't necessarily eql, so using them in defconstant is touchy with SBCL.

Full log is here: http://report.quicklisp.org/2020-02-12/failure-report/cl-zyre.html#zyre_zyredir

[jesseoff]

I pushed an update to get that should change it to a defparameter. Thanks.