meson: use booleans for conf.set and drop unecessary conditionals

Using conf.set() with a boolean argument does the right thing:
either #ifdef or #undef. This means that conf.set can be used unconditionally.

Previously I used '1' as the placeholder value, and that needs to be changed to
'true' for consistency (under meson 1 cannot be used in boolean context). All
checks need to be adjusted.

shell-completion/bash/meson.build

@@ -42,7 +42,7 @@ if bashcompletiondir != 'no'
                 ]
 
         foreach item : items
-                if item[1] == '' or conf.get(item[1], 0) == 1
+                if item[1] == '' or conf.get(item[1], false)
                         install_data(item[0],
                                      install_dir : bashcompletiondir)
                 endif