meson: Create bindir and libdir variables

This is to avoid using the construct 'join_paths(prefix, get_option(...))'
everywhere in the meson files. It's better to settle the paths question
once and for all at the beginning.

Signed-off-by: Arnaud Rebillout <arnaud.rebillout@collabora.com>

src/utils/meson.build

@@ -82,6 +82,6 @@ if cc.has_header('sys/soundcard.h')
     output : 'padsp',
     configuration : cdata,
     install : true,
-    install_dir : join_paths(prefix, get_option('bindir')),
+    install_dir : bindir,
   )
 endif