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GNUInstallDirs
Define GNU standard installation directories
Provides install directory variables as defined by the GNU Coding Standards.
Result Variables
Inclusion of this module defines the following variables:
CMAKE_INSTALL_<dir>
DESTINATION options of install() commands for the corresponding file type. 
  CMAKE_INSTALL_FULL_<dir>
CMAKE_INSTALL_<dir> value. If the value is not already an absolute path, an absolute path is constructed typically by prepending the value of the CMAKE_INSTALL_PREFIX variable. However, there are some special cases as documented below. 
  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 (
liborlib64orlib/<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) 
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.
Special Cases
The following values of CMAKE_INSTALL_PREFIX are special:
/
For <dir> other than the SYSCONFDIR and LOCALSTATEDIR, the value of CMAKE_INSTALL_<dir> is prefixed with usr/ if it is not user-specified as an absolute path. For example, the INCLUDEDIR value include becomes usr/include. This is required by the GNU Coding Standards, which state:
/usr will be a symbolic link to /. 
  /usr
<dir> equal to SYSCONFDIR or LOCALSTATEDIR, the CMAKE_INSTALL_FULL_<dir> is computed by prepending just / to the value of CMAKE_INSTALL_<dir> if it is not user-specified as an absolute path. For example, the SYSCONFDIR value etc becomes /etc. This is required by the GNU Coding Standards. 
  /opt/...
<dir> equal to SYSCONFDIR or LOCALSTATEDIR, the CMAKE_INSTALL_FULL_<dir> is computed by appending the prefix to the value of CMAKE_INSTALL_<dir> if it is not user-specified as an absolute path. For example, the SYSCONFDIR value etc becomes /etc/opt/.... This is defined by the Filesystem Hierarchy Standard.
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Licensed under the BSD 3-clause License.
 https://cmake.org/cmake/help/v3.6/module/GNUInstallDirs.html