Implement support for multiarch as specified here:

  http://wiki.debian.org/Multiarch
  https://wiki.ubuntu.com/MultiarchSpec

Detect the <arch> part of <prefix>/lib/<arch> from the implicit library
search path from each compiler to set CMAKE_<lang>_LIBRARY_ARCHITECTURE.
Define CMAKE_LIBRARY_ARCHITECTURE using one of these values (they should
all be the same).  Teach the find_library and find_package commands to
search <prefix>/lib/<arch> whenever they would search <prefix>/lib.