stdenv: fix self.cc check

Since ENV.cc/cxx are accessors to ENV, any values being passed through
are coerced into strings, which means that a nil `ENV.cc` is actually
an empty string - which isn't considered to be false in Ruby.

Fixes mistydemeo/tigerbrew#161.
Fixes mistydemeo/tigerbrew#135.
default_cc: locate already returns Pathnames
default_compiler: recognize gcc-4.0
stdenv: set correct compiler symbol for gcc 4.0
mitmproxy: switch to pillow; fixes #25522
minizip: needs newer autotools

Fixes mistydemeo/tigerbrew#164.
tor: needs newer openssl on < Snow Leopard

Fixes mistydemeo/tigerbrew#163.
tor: fix conditionals