Changes default macosx compiler to clang.

The default gcc version provided by current Xcode versions doesn't work.
Alternatively macport's gcc-4.7 is known to work too, but that is less
common for macosx users.
lazy operand sem'checking (beware)
make some tests green
fixes for the new overloading resolution
made some tests green
Merge pull request #349 from gradha/pr_switches_default_macosx_compiler_to_clang

Changes default macosx compiler to clang.
use clang for macosx in buildsh.tmpl
Merge branch 'master' of github.com:Araq/Nimrod