rtags: fix for clang/libc++ 4

This also fixes a missing header in the SDK that rtags needs to work
properly. The underlying cause is that C++ headers got shuffled around a
lot in libc++ 3.8 (I believe) and became more standards-compliant, which
led to a lot of C-compatible passthrough header files being added to it
like math.h, which defines some C++-compatible versions of standard
functions like signbit, while #include_next'ing the system math.h. In
this case, including the SDK was stuffing another math.h in front of the
libc++ shim, which led to all sorts of mysterious failures.