Initial CMake build for Linux
CMake build support for OSX and Win32

TODO: "make prep" code generation targets.
Add "prep" code generation step to CMake builds
CMake build basic support for MSVC
CMake: fix Win32 MinGW/MSVC detection
CMake Win32 MinGW: remove unused libm dependency
CMake: rename CMake project to "rebol3"

Some generators (such as Eclipse) work better with distinct names for
project and executable targets.
CMake: use r3-make for bootstrapping

Use "r3-make" (not "rebol3") for code generation in the "prepare" phase,
like in the mainline makefiles.
CMake: fix dependency from "r3" on "prepare" for MSVS

The MS Visual Studio generator does not take dependencies between files
in different targets into account. It creates one "project" for each
target, but does not life file-level dependencies to project-level
dependencies.

To fix this, we add an explicit dependency from "r3" on "prepare" when
targeting MSVC.
Make r3-make lookup more robust and configurable
Add f-dtoa.c to source list
CMake: don't set REB_API flag

We currently don't build the standalone R3 lib, so don't set the flag
which indicates we do.