Vendored libevent 2.1.12 triggers 6 obsolete-macro warnings under modern autoconf (2.72+)

LibMR vendors libevent 2.1.12-stable (deps/libevent) and runs autoreconf on it at build time. With autoconf 2.72+ (current on Alpine 3.24 and most 2025+ distros), this prints six obsolete-macro warnings on every build:

• AC_HAVE_LIBRARY (configure.ac:351)
• AC_HEADER_TIME (configure.ac:357)
• AC_LANG_C + AC_TRY_LINK (via ACX_PTHREAD, m4/acx_pthread.m4)
• AC_PROG_GCC_TRADITIONAL
• AC_OUTPUT with arguments

Harmless, but noisy in CI. Note that no stable libevent fixes this: 2.1.13-stable (July 2026) still carries all the old macros — the modernized configure.ac (AX_PTHREAD etc.) only exists on the 2.2 line, which is still alpha (2.2.2-alpha).

Filing this mostly as a tracking issue: once a stable 2.2 ships, bumping the vendored libevent would silence the warnings for every downstream builder (we hit this building RedisTimeSeries as a redis 8.8.0 module). Observed with RedisTimeSeries v8.8.0 / current LibMR on Alpine 3.24.

Track the vendored libevent copy under deps/libevent, including configure.ac and m4/acx_pthread.m4. Revisit when a stable libevent 2.2 release is available, then update the vendored dependency and run autoreconf on Alpine 3.24 or another autoconf 2.72+ environment. Done means the build completes without the six obsolete-macro warnings.