Allow readonly PERL_INLINE_DIRECTORY

[epa]

Currently Inline requires that the directory specified by PERL_INLINE_DIRECTORY be writable. If it's not writable then it fails with "can't find an appropriate DIRECTORY", even if all the compiled code it needs is already there.

To make it easier to use Inline with read-only file systems, or in processes running with less privilege and without write permission, it should continue as long as the directory is readable. Only if it later turns out that something needs to be rebuilt should it die if write permission is missing.

I think that when automatically working out the best place to put the _Inline directory it can continue to check for write access. But when a directory is explicitly specified using Config or with the environment variable it should just be used, whether writable or not.

[mohawk2]

This seems like it could/should be dealt with at same time as #64.