Update OS X build instructions

[eschnett]

I have written a second set of build instructions for OS X that are based on MacPorts. They are significantly simpler than building clang and libcxx manually. I attach the patch.

[eschnett]

It seems that patches cannot be attached; only images can be attached. I attach my patch inline.

{{{
diff --git a/README.rst b/README.rst
index 2832b4a..f8bad42 100644
--- a/README.rst
+++ b/README.rst
@@ -96,6 +96,7 @@ Linux

 $ cmake -DBOOST_ROOT=/your_boost_directory \
      -DHWLOC_ROOT=/your_hwloc_directory \

•     -DCMAKE_INSTALL_PREFIX=/where_hpx_should_be_installed \
    [other CMake variable definitions] \
    /path/to/hpx/source/tree
  

@@ -120,8 +121,12 @@ Linux

to build and install the examples.

-Mac OSX

+OS X (Mac)
+----------
+
+The standard system compiler on OS X is too old to build HPX. You will
+have to install a newer compiler manually, either Clang or GCC. Below
+we describe two possibilities:

1. Install a recent version of LLVM and Clang.
   In order to build hpx you will need a fairly recent version of Clang
   @@ -198,6 +203,30 @@ Mac OSX

to build and install the examples.

+Alternatively, you can install a recent version of gcc as well as all
+required libraries via MacPorts:
+
+1) Install MacPorts http://www.macports.org/
+
+2) Install Boost, CMake, gcc 4.8, and hwloc:
+

• $ sudo port install boost
• $ sudo port install gcc48
• $ sudo port install hwloc

• You may also want:

• $ sudo port install cmake
• $ sudo port install git-core

+3) Make this version of gcc your default compiler:
+

• $ sudo port install gcc_select
• $ sudo port select gcc mp-gcc48

+4) Build HPX as described above in the ``Linux'' section.
+
+
Windows

}}}