[tlbuild] Building on the Macintosh (Part 2)

[Akira Kakuto]

Hi Karl,

> The patch merely unconditionally adds --std=c++11 to OBJCXXFLAGS.
> Is that really what we want to do? -k

I think that is relevant only for compiling
XeTeXFontMgr_Mac.mm.
so, that may be safe.

Thanks,
Akira