Build: Various improvements to install/pkg system - GNUInstallDirs: any directory variable can now reference any other directory variable by including its name in angle brackets (<>). - Changed the documentation of the directory variables in BUILDING.md accordingly. This commit also includes some formatting tweaks to that section (using boldface for directory names, as is our convention.) - Changed the package scripts such that they use CMAKE_INSTALL_DATAROOTDIR rather than CMAKE_INSTALL_DATADIR. - We no longer override the install dir. defaults on Windows unless performing an official build. It may be useful, for instance, to use the GNU defaults when installing into an MSYS environment.