implementations.  Details:
  * replace 'link_shared_object()', 'link_shared_lib()', and
    'link_executable()' with 'link()', which is (roughly)
    the union of the three methods it replaces
  * in all implementation classes (UnixCCompiler, MSVCCompiler, etc.),
    ditch the old 'link_*()' methods and replace them with 'link()'
  * in the abstract base class (CCompiler), add the old 'link_*()'
    methods as wrappers around the new 'link()' (they also print
    a warning of the deprecated interface)

Also increases consistency between MSVCCompiler and BCPPCompiler,
hopefully to make it easier to factor out the mythical WindowsCCompiler
class.  Details:
  * use 'self.linker' instead of 'self.link'
  * add ability to compile resource files to BCPPCompiler
  * added (redundant?) 'object_filename()' method to BCPPCompiler
  * only generate a .def file if 'export_symbols' defined