If we are using the Apple toolchain on MacOS X, we must use the
newer runtime and not the old GCC 4.2 fork, in order to get the
C++11 library features (such as std::shared_ptr).
v2: incorporate Roland's nitpicks (I hope)
v3: It was decided that it's better to change the default behavior
v4: make sure the CMake syntax is correct even if the value of the
variable contains spaces (if it contains double quotes it's a
different matter, but that's pretty much a corner case, IMHO)
v5: properly escape backslash and double quote characters in the cmake
syntax
