Improved SRTP negotiation capabilities

Ignore-this: 66293e3b93c319cf43da4509a56f12cc

    - If both endpoints have it set to optional, SRTP will be used
    - Generate proper error message if incoming stream demands SRTP
      but it's not locally enabled
    - Generate proper error message if incoming stream doesn't offer SRTP
      but it's not locally mandatory

darcs-hash:20100511103733-56b4d-a3b39ce8ee9fd74400af679b46cf3f871c2c0288
Set compiler and linker in user.mak file for Snow Leopard

Ignore-this: 1403b17d0ac403e93de93c3498dd6eed

darcs-hash:20100511123442-56b4d-204e63f7d7a1d4981e9e16f8ec54dcde592da8bd