Only build libchrome on Linux.

I'm not sure what the brillo plan is, but this currently doesn't build
on Mac OS because there's a #define __linux__ 1 in build/build_config.h;
a lie which makes libc++ understandably unhappy. That whole block
of #defines looks suspicious, so rather than touch it let's just not
even try to build for the Mac for now.

Change-Id: Ie50f2087376e8563f67db8b81dc7509d77368fb8
1 file changed