meson: Fix module_dir configuration (v2)

meson.build has code to set the module_dir variable to
${libdir}/xorg/modules if the module_dir option string is empty.
However, this has several problems:

1. The variable is only used for an unused @moduledir@ substitution in
   the man page. The rule for xorg-server.pc uses option('module_dir')
   directly instead.
2. The 'module_dir' option has a default value of 'xorg/modules' so the
   above rule doesn't do anything by default.
3. The xorg-server.pc rule uses ${exec_prefix}/option('module_dir'), so
   the effect of #2 is that the default moduledir is different between
   autoconf and meson. E.g. if ${prefix} is /X, then you get

     autoconf: moduledir=/X/lib/xorg/modules
     meson:    moduledir=/X/xorg/modules

Fix this by using the module_dir variable when generating xorg-server.pc, and by
using join_paths() to assign module_dir unconditionally.

v2: Keep the 'xorg/modules' default path, but use join_paths() unconditionally (Thierry Reding)

Signed-off-by: Aaron Plattner <aplattner@nvidia.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>