This PR brings the Mac OSX build instructions up to date slightly. (They
currently refer to facts "...as of November 2014...")
- Remove specific OS version number from the title as it is out of date
with respect to filename.
- Nonetheless don't change filename for the sake of not breaking
incoming links.
- Update C++ language version to C++14, which I believe is what is
currently required, based on other platform documentation.
- Bump versions of C++ compilers that seem to be current. I expect the
exact values of these version numbers aren't crucial but maybe good for
the reader calibrating a vague sense of whether their compiler is in the
right ballpark.
- Add `lld` to the homebrew clang instructions, because homebrew changed
the way they package llvm tools, spinning the linker off into its own
package.
now if use g++4.9 as compiler flag on OSX, it will produce error message
that
```
CMake Error at CMakeLists.txt:2 (project):
The CMAKE_CXX_COMPILER:
g++-4.9
is not a full path and was not found in the PATH.
Tell CMake where to find the compiler by setting
either the environment variable "CXX" or the CMake cache entry
CMAKE_CXX_COMPILER to the full path to the compiler, or to the
compiler name if it is in the PATH.
```
