.present returns std::nullopt if the optional argument is not given by the
user -- as long as a .default_value is not defined. With a .default_value,
.present cannot be used to determine if a value is user-provided or the
default.
.is_used fills that role and only returns true if the argument was passed
by the user.
Signed-off-by: Sean Robinson <sean.robinson@scottsdalecc.edu>
The default behavior with optional arguments is to allow only a single use
per invocation. One alternative is to use .nargs, but this requires
previously knowing, and limiting, the quantity of values. The .append
method removes the restriction on repeats for a single Argument.
Signed-off-by: Sean Robinson <sean.robinson@scottsdalecc.edu>
This change also explicitly lists the source files
for CMake. This is because `GLOB` does not remove
old files when switching branches in Git, and
`CONFIGURE_DEPENDS` will add unstaged files after
stashing.
See also: https://cmake.org/cmake/help/latest/command/file.html#glob
