SERVER-3364 Add macros for putting visibility and calling convention declarations on declarations.

Also SERVER-5650.

MONGO_COMPILER_API_EXPORT Puts the "export visibility" declaration on a type,
    function or variable.

MONGO_COMPILER_API_IMPORT Puts the "import from library" declaration on a type,
    function or variable.

MONGO_COMPILER_API_CALLING_CONVENTION Labels a function declaration with its
    calling convention.