Configure: Check source and build dir equality a little more thoroughly

Configure: Check source and build dir equality a little more thoroughly

'absolutedir' does a thorough job ensuring that we have a "real" path
to both source and build directory, unencumbered by symbolic links.
However, that isn't enough on case insensitive file systems on Unix
flavored platforms, where it's possible to stand in, for example,
/PATH/TO/Work/openssl, and then do this:

    perl ../../work/openssl/Configure

... and thereby having it look like the source directory and the build
directory aren't the same.

We solve this by having a closer look at the computed source and build
directories, and making sure they are exactly the same strings if they
are in fact the same directory.

This is especially important when making symbolic links based on this
directories, but may have other ramifications as well.

Fixes #12323

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/12337)