build: remove clang and gcc_version gyp defines

The clang and gcc_version defines were used to:

1. Conditionally enable -fvisibility=hidden, a switch introduced in
   gcc 4.0.  Because 4.2 is the minimum supported version, we can
   make it unconditional.  People that want to build with older
   versions, can use '-Dvisibility='.

2. To conditionally enable -Wno-dollar-in-identifier-extension,
   an Apple extension that suppressed warnings about sigils in
   dtrace probe names.  Dtrace probes were removed in cb514002.

R=@saghul