This teaches CMake to detect implicit link information for C, C++, and
Fortran compilers.  We detect the implicit linker search directories and
implicit linker options for UNIX-like environments using verbose output
from compiler front-ends.  We store results in new variables called

  CMAKE_<LANG>_IMPLICIT_LINK_LIBRARIES
  CMAKE_<LANG>_IMPLICIT_LINK_DIRECTORIES

The implicit libraries can contain linker flags as well as library
names.