Merged a few fixes for non-posix systems from github.com/Siesh1oo/yosys
(see https://github.com/cliffordwolf/yosys/pull/28)
Merged addition of SED makefile variable from github.com/Siesh1oo/yosys
(see https://github.com/cliffordwolf/yosys/pull/28)
OSX compatible creation of stdcells.inc, using code from github.com/Siesh1oo/yosys
(see https://github.com/cliffordwolf/yosys/pull/28)