Add system property to disable spec macro instrumentation

Macro specs are checked at macro expansion time and it is not currently possible to disable this. One place where this is an issue is while compiling the spec.alpha library itself. There may be other cases where it is useful to disable these macro spec checks as well for the purposes of debugging something.

The attached patch adds a system property "clojure.spec.skip-macros" which defaults to false but can be set to true to skip the macro checks.

Patch: skip-macros-2.patch

Screened by Stu, who resisted the temptation to bikeshed on what the property should be named. Verified that spec.alpha builds correctly when the property is set.