Add CHECK_SHLIB_SKIP_RPATHS support

[Jonathan Perkin <jperkin%joyent.com@localhost>]

I've been recently digging into an issue where some of our binaries
were pulling in the wrong GCC libraries, and avoiding detection
because these libraries are in the system paths (so you can't just use
e.g. CHECK_WRKREF_EXTRA_DIRS).

To help avoid this I've come up with support for
CHECK_SHLIB_SKIP_RPATHS which allows you to specify paths that should
be skipped when resolving libraries:

  https://gist.github.com/jperkin/b292bf9049c3f9d91f61aff6b8121706

So in my case if I specify something like this:

  CHECK_SHLIB_SKIP_RPATHS=	^/usr/lib.*/lib(gcc|ssp)

then I get warned about any packages which are pulling in libgcc et al
from the platform rather than correctly linking against pkgsrc gcc.

Look ok?

-- 
Jonathan Perkin  -  Joyent, Inc.  -  www.joyent.com