Add Emscripten and VS2010-specific customizations to PNaCl build.

Visual Studio doesn't come with the inttypes.h header, but it doesn't seem to be necessary to build that file, at least on VS2010. Also, PNaCl (pnacl-llc) doesn't build on Windows due to no pthreads support, but we can just remove PNaCl tools from the build altogether, so I put those tools behind a `ENABLE_PNACL` flag which is off for our builds.