Fix for meson >=0.62.0

Index: meson.build
--- meson.build.orig	2022-08-23 12:52:30 UTC
+++ meson.build
@@ -55,9 +55,6 @@ stable = (stability_version_number % 2 == 0)
 stability_version_number = (major_version != 0 ? minor_version : micro_version)
 stable = (stability_version_number % 2 == 0)
 
-conf.set10('BABL_UNSTABLE', not stable, description:
-  'Define to 1 if this is an unstable version of BABL.')
-
 conf.set       ('BABL_MAJOR_VERSION',    '@0@'.format(major_version))
 conf.set       ('BABL_MINOR_VERSION',    '@0@'.format(minor_version))
 conf.set       ('BABL_MICRO_VERSION',    '@0@'.format(micro_version))