configure: Add option to specify global defaults directory.
Previously, this was only (partially) possible by redefining the macro
GLOBAL_DEFAULTS_SUBDIR. This told Window Maker to look for the global
config files in a particular subdirectory of SYSCONFDIR.
However:
* This is undocumented.
* GLOBAL_DEFAULTS_SUBDIR is ignored when installing the config files. They
are always installed to SYSCONFDIR/WindowMaker.
To solve these issues, we add a "--with-defsdatadir" option to configure
which allows a user to specify the global defaults directory.