Define GNU standard installation directories

Provides install directory variables as defined for GNU software:

Inclusion of this module defines the following variables:

CMAKE_INSTALL_<dir>      - destination for files of a given type
CMAKE_INSTALL_FULL_<dir> - corresponding absolute path

where <dir> is one of:

BINDIR           - user executables (bin)
SBINDIR          - system admin executables (sbin)
LIBEXECDIR       - program executables (libexec)
SYSCONFDIR       - read-only single-machine data (etc)
SHAREDSTATEDIR   - modifiable architecture-independent data (com)
LOCALSTATEDIR    - modifiable single-machine data (var)
LIBDIR           - object code libraries (lib or lib64 or lib/<multiarch-tuple> on Debian)
INCLUDEDIR       - C header files (include)
OLDINCLUDEDIR    - C header files for non-gcc (/usr/include)
DATAROOTDIR      - read-only architecture-independent data root (share)
DATADIR          - read-only architecture-independent data (DATAROOTDIR)
INFODIR          - info documentation (DATAROOTDIR/info)
LOCALEDIR        - locale-dependent data (DATAROOTDIR/locale)
MANDIR           - man documentation (DATAROOTDIR/man)
DOCDIR           - documentation root (DATAROOTDIR/doc/PROJECT_NAME)

Each CMAKE_INSTALL_<dir> value may be passed to the DESTINATION options of install() commands for the corresponding file type. If the includer does not define a value the above-shown default will be used and the value will appear in the cache for editing by the user. Each CMAKE_INSTALL_FULL_<dir> value contains an absolute path constructed from the corresponding destination by prepending (if necessary) the value of CMAKE_INSTALL_PREFIX.