Commented out apsp_johnson '-fno-delete-null-pointer-checks' flag for Mac OSX build

This fix will support Xcode 5.1 clang build on Mac OSX (#258).
I confirmed that all test work correctly on Mac OSX.
And I think that the following notice(original comment) in "src/apsp_johnson/src/CMakeLists.txt" should be fixed in C/C++ source level.
<pre>
# FIXME: The following patch is required by apsp_johnson_boost_wrapper.cpp.
# With -fdelete-null-pointer-checks optimization, the assignments
# to Edge bundled properties doesn't occur.
</pre>