Add feature for linking against static libc/msvcrt.

* CMakeLists.txt now handle a variable CC0_USE_STATIC_CRT to indicate
  whether to link against static C/C++ runtime library, on UNIX-based
  platforms it is libc and libstdc++, while on Windows it is Microsoft
  VC++ Redistributes(msvcrt.dll, msvcrtd.dll, etc.).

* When setting compiler variables always honor what is already there,
  so either append or replace the flags you want to modify, instead of
  setting it as a whole.

* Also I handle 5 standard build types in CMake - None, Debug, Release,
  RelWithDebInfo, MinSizeRel. It is defined in standard CMake variable
  CMAKE_BUILD_TYPE.

* P.S., To set CMake variables, pass -D<VarName>=<Value> to cmake
  command line, e.g.
      cmake -DCMAKE_BUILD_TYPE=Release -DCC0_USE_STATIC_CRT=TRUE ..
  It will build a release version linking against static CRT.