Explicitly export libuv symbols if gcc >= 4.
Explicitly export libuv symbols if gcc >= 4.

Only export symbols that are part of the libuv API, hide everything else.

Prevents symbol clashes in applications and libraries that depend on libuv and
speeds up link times to boot.

unknownbrackets/maxcso,urbit/archaeology2,unknownbrackets/maxcso,unknownbrackets/maxcso,urbit/archaeology2,urbit/archaeology2,urbit/archaeology2,unknownbrackets/maxcso,urbit/archaeology2,unknownbrackets/maxcso,unknownbrackets/maxcso,urbit/archaeology2,unknownbrackets/maxcso,urbit/archaeology2,urbit/archaeology2,urbit/archaeology2