Icarus-verilog-0.9.5 crashes clang or llvm into an assertion
error when either of those two binaries is used as the linker.
Add a command to set `ENV['LD']`. It also works for XCode-only.
Remove both the fails_with because it works now.
Add a separate line to `make installdirs` that fixes a build error
where a directory does not exist.
Tested on ML with XCode-4.4.1 using clang and llvm.
FixesHomebrew/homebrew#13650ClosesHomebrew/homebrew#14138.
Signed-off-by: Jack Nagel <jacknagel@gmail.com>