During configuration some of the values for CMake properties (compiler
flags etc.) may be changed either by the user (through command line
parameters or option files), or by the configuration script itself
(setting more aggressive options or detailed debugging for instance).
This change writes many of the relevant values back into the cache so
they are available for introspection (using e.g. ccmake) or when the
rerunning the configuration without specifying all options.
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)
When checking out from source code, the paths that were used to
configure the project should be carried on to the installed binary,
so the same libraries are used without much effort.
By using RUNPATH instead of (just) RPATH, it can be overridden with
LD_LIBRARY_PATH if the user so desires.
