build: Fix USER_DIR_MODE value in config.h

meson defines `USER_DIR_MODE` with a raw octal value to be used as
the default permissions when creating the user's configuration
directory.

However, meson does not support raw octal values[0], so the define
misses the initial `0` value. Due to this, the directory is created
with wrong permissions.

This has been changed to use the octal value as a string in meson,
so the definition has the proper value.

Fixes #49

[0] mesonbuild/meson#2047

meson.build

@@ -44,7 +44,9 @@ foreach define: set_defines
   config_h.set_quoted(define[0], define[1])
 endforeach
 
-config_h.set('USER_DIR_MODE', 0700,
+# meson does not support octal values, so it must be handled as a
+# string. See: https://github.com/mesonbuild/meson/issues/2047
+config_h.set('USER_DIR_MODE', '0700',
              description: 'Permissions for creating the user\'s config, cache and data directories')
 
 # compiler flags