Add function to detect linker version

The linker that is used (ld vs. gold for instance) is often hidden
when using the compiler as a front-end (to include the correct
runtime libraries).

These functions enables us to probe the linker version and reports
its finding in the log.
Print linker version to log
Parse RHEL5 linker strings better
Merge pull request #278 from rolk/278_linkver

Print linker version to log when configuring