This is enabled for platforms/compilers that support the -MMD
option, which creates a dependency file named out.d when compiling
to a file named out.o, only including local, not system headers.
The additional -MP option adds phony targets for all included
headers, to avoid having make error out due to "no rule to make
target foo.h" if a header is renamed/removed.
This makes sure that all the right files are rebuilt if a header
is changed, avoiding the need for a full rebuild if only a few
files are affected.
Don't enable threading globally since the MSVC/ARM build target
(neither windows phone nor windows rt) doesn't support the normal
windows threading functions.
The library uses this define to enable OS X specific functionality
in general, even if none of those snippets actually have been
strictly required so far.
