Clang aims to be compatible with GCC when it comes to command-line
parameters. Where we enable functionality based on the presence of
a GCC-compiler, we can use the same functionality with Clang.
This patch introduces a new variable CXX_COMPAT_GCC which is true if
the compiler handles the same options as GCC, and this variable is
subsequently used in tests instead of CMAKE_COMPILER_IS_GNUCXX (which
remains if we need to test if we really have GCC, e.g. for 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.
Some distributions modify -dumpversion so that it only returns major
and minor numbers, not the patch level. This is usually OK for
determining features, but when debugging the output log, we really
want the full number.
