":safe-sbcl" feature

[thephoeron]

A few issues with the quick-and-dirty hack to support the new implementation of backquotes in SBCL 1.2.2 were reported by @csrhodes:

File-Compiling: there needs to be an eval-when (:compile-toplevel :execute) around the sbcl version test and *features* frobbing, otherwise file-compiling the code will not have the expected effect.

Shared Namespace of *features*: use a package-internal symbol for the reader *features*, rather than the :safe-sbcl keyword

Long-Term Support of Version Test: the code to test the sbcl version will fail with the release of SBCL 1.2.10, either use sb-ext:assert-version->= or test for the representation of backquote directly

[thephoeron]

That covers File Compiling and Long-Term Support of Version Test...