Build: Minor tweaks to GNUInstallDirs defaults
It isn't actually necessary to specify `CMAKE_INSTALL_DEFAULT_MANDIR`
for our official build. Because `CMAKE_INSTALL_DEFAULT_DATAROOTDIR` is
blank for the official build, the default of "<DATAROOTDIR>/man" will
resolve to "man".
For the same reason, this commit changes the specification of
`CMAKE_INSTALL_DEFAULT_DOCDIR` and `CMAKE_INSTALL_DEFAULT_JAVADIR` in
the official build to be dependent on the data root directory (mainly to
make it obvious what we're doing.)
This commit also tweaks the example CMake command line in the directory
variable documentation so that it shows the correct location of the
CMake argument.
2 files changed