Make the processing of build.info files more aware of absolute dirs

There were cases where some input was absolute, and concatenating it
to the diretory to the source or build top could fail spectacularly.
Let's check the input first to see if it's absolute.

And while we're on the subject of checking if a file or dir spec is
absolute using file_name_is_absolute() has its own quirks on VMS,
where a logical name is considered absolute under most circumstances.
This is perfectly correct from a VMS point of view, but when parsing
the build.info files, we want single word file or directory names to
only be checked syntactically.  A function isabsolute() that does the
right thing is the solution.

Reviewed-by: Rich Salz <rsalz@openssl.org>