Fix Genera's special case to skip /usr/share/common-lisp

OS-COND evaluates its clauses at compile time and only includes the relevant clause.
Change the clause in XDG-DATA-DIRS for Genera to check whether we're on a VLM running
on macOS at runtime. Otherwise, the binary we distribute with Genera will either
include /usr/share/common-lisp or it won't depending on which machine I use to compile.
9 jobs for !171 with genera-usr-share-fix-redux in 41 minutes and 44 seconds (queued for 4 seconds)
latest detached
Status Name Job ID Coverage
  Build
passed Build ASDF #13255

00:03:08

passed Build docs #13256

00:06:37

 
  Regression
passed ABCL 1.7.1 regression tests #13258

00:38:35

failed ABCL regression tests #13257
allowed to fail

00:33:53

passed CCL regression tests #13259

00:09:03

passed CLISP regression tests #13260

00:07:57

passed CMUCL regression tests #13261

00:08:32

passed ECL regression tests #13262

00:07:17

passed SBCL regression tests #13263

00:06:36

 
Name Stage Failure
failed
ABCL regression tests Regression
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