lyaml build script detects Darwin using `uname -s` and changes
build logic so lyaml package can not be built on macos.

This patch uses fakeuname host tool to redefine `uname -s` output
and fix build on macos.

Signed-off-by: Sergey V. Lobanov <sergey@lobanov.in>

lyaml provides a Lua binding for libyaml

Signed-off-by: Mathew McBride <matt@traverse.com.au>