Merge #773: Fix some compile problems on weird/old compilers.

1309c03c45beece646a7d21fdb6a0e3d38adee2b Fix some compile problems on weird/old compilers. (Gregory Maxwell)

Pull request description:

  The visibility attribute is a GCC 4+ feature.
  GCC 2.95 also warns about the unsigned/signed comparision.

ACKs for top commit:
  real-or-random:
    ACK 1309c03c45beece646a7d21fdb6a0e3d38adee2b I inspected the diff

Tree-SHA512: b5a5175416b67b2619f68ad82a208052ad678955e59c2f3457799abd1dd6fd817c40f6bc2941b2bda207c6f58ad0fbe46221a2f92b726e824702c4c0b177377c