meson: prepare 0.2.18-rc3

Bump API to 4.12.0, ABI to 15:0:11 (soversion 4 unchanged,
binary-compatible with 0.2.15/0.2.16/0.2.17 and rc2).  New interfaces
since rc2: rist_recovery_depth_set() with the
RIST_RECOVERY_DEPTH_MIN/DEFAULT/MAX macros, and the additive stats
fields (RIST_STATS_VERSION 3: profile / seq_bits / advanced_active).

Changes since rc2:
  - Advanced-profile correctness fixes (>64k sequence gaps, ring-index
    masking in receiver_mark_missing, expected-next-seq wrap mask,
    payload-only byte accounting, sequence-based duplicate detection,
    32-bit-safe merge-mode pairing).
  - Default profile is now Advanced with Main interoperability (TR-06-3
    Section 9); risttunnel follows RIST_DEFAULT_PROFILE.
  - Tunable Advanced recovery-ring depth via ?recovery-depth=.
  - Advanced-profile profile/framing visibility in the stats API and the
    Prometheus exporter.

meson.build

@@ -17,9 +17,9 @@ cdata = configuration_data()
 #If any interfaces have been added, removed, or changed since the last update, increment current, and set revision to 0.
 #If any interfaces have been added since the last public release, then increment age.
 #If any interfaces have been removed or changed since the last public release, then set age to 0.
-librist_abi_current = 14
+librist_abi_current = 15
 librist_abi_revision = 0
-librist_abi_age = 10
+librist_abi_age = 11
 librist_soversion = librist_abi_current - librist_abi_age
 librist_version = '@0@.@1@.@2@'.format(librist_abi_current - librist_abi_age, librist_abi_age, librist_abi_revision)
 
@@ -30,7 +30,7 @@ librist_version = '@0@.@1@.@2@'.format(librist_abi_current - librist_abi_age, li
 #PATCH not used (doesn't make sense for API version, remains here for backwards compat)
 
 librist_api_version_major = 4
-librist_api_version_minor = 11
+librist_api_version_minor = 12
 librist_api_version_patch = 0
 
 librist_src_root = meson.current_source_dir()