CMake: Silence LOCATION-related warning.

We need to refactor our code to avoid it eventually,
see GitHub #658.
CMake: Refactor druntime/Phobos compilation.

Having the compilation step separate from the actual library
creation lets us reuse it for the new static unittest builds.
Make -defaultlib/-debuglib override instead of append; deprecate -nodefaultlib.

This is necessary as the only point of having -defaultlib
and -debuglib is to enable the user to customize the defaults
set in the configuration, for example to avoid linking to
Phobos.

GitHub: Fixes #416.
GitHub: Fixes #601.
CMake: Fix typo.

The variable was empty when not building with MSVC anyway.
Paste flags from config file in front of command line arguments.

This way, the user can override any defaults defined in the
config file.
Remove rebuild config file.

rebuild was somewhat widely used in the old D1 days, but now
is dead. Proof: The file was broken for a long time (note the
double -I-I), but not a single person complained. So even if
anybody still uses it, they are using a custom config file
already anyway.
Use single test runner executable for unit tests.

This is not only much cleaner than the old hack that depended
on the tested modules not being linked in a second time from
the druntime/Phobos static libraries, but the hack obviously
also doesn't work for shared libraries.