icebox install on macOS requires GNU sed

[ahooper]

To install icestorm on macOS 10.15.6 I had to install gsed with homebrew and change sed to gsed in icetools/icestorm/icebox/Makefile
gsed parses the -i option differently.

Also yosys, iverilog(icarus-verilog), and verilator are available in homebrew, thus redundant in icetools on Darwin.

[adimitris]

Alternatively, changing all the sed -i lines in icebox/Makefile from sed -i '...' to sed -i '' '...' should complete the installation successfully on macOS 10.15.