GNUInstallDirs.cmake: Various improvements

- Integrate
  c07bba2730 (diff-1e2deb5301e9481203102fcddd1b2d0d2bf0ddc1cbb445c7f4b6414a3b869ce8)
  so that the default man directory is <CMAKE_INSTALL_PREFIX>/share/man
  on FreeBSD systems.

- For good measure, integrate
  f835f189ae
  so that the default info directory is
  <CMAKE_INSTALL_PREFIX>/share/info on FreeBSD systems, even though we
  don't use that directory.

- Automatically set the CMake variable type to PATH for any
  user-specified CMAKE_INSTALL_*DIR variables.

Addresses concerns raised in #326, #346, #648

Closes #648

BUILDING.md

@@ -605,15 +605,6 @@ brackets, then its final value will depend on the final value of that other
 variable.  For instance, the default value of `CMAKE_INSTALL_MANDIR` is
 **\<CMAKE\_INSTALL\_DATAROOTDIR\>/man**.
 
-NOTE: If setting one of these directory variables to a relative path using the
-CMake command line, you must specify that the variable is of type `PATH`.
-For example:
-
-    cmake -G"{generator type}" -DCMAKE_INSTALL_LIBDIR:PATH=lib {source_directory}
-
-Otherwise, CMake will assume that the path is relative to the build directory
-rather than the install directory.
-
 
 Creating Distribution Packages
 ==============================