c-accel: fix strdup on glibc with -std=c99

strdup is POSIX, not ISO C99, so glibc's <string.h> only exposes it when
_POSIX_C_SOURCE >= 200809. With -std=c99 (strict mode) gcc otherwise
treats strdup as an implicit-int function, which on 64-bit Linux
truncates the returned pointer to int and yields warnings + likely
crashes. macOS happens to declare strdup unconditionally so the issue
only surfaces on the srht.bigb.es Ubuntu builder.

Define _POSIX_C_SOURCE at the top of c_runtime.c before any include.

c-accel: descriptor -> C plan compiler (bd-mq7)

First C source for the pb.c_runtime module. Compiles a finalized Lua
descriptor into an opaque pb_plan userdata, the foundation that
bd-ra6's encode/decode entry points will walk.

What the plan carries (per docs/specs/c_accel_strategy.md):

  * Per-field records: field_number, wire_type, kind, repeated/packed/
    optional flags, pre-encoded tag bytes (varint, up to 5 bytes),
    sub_plan_idx (1-based into the sub-plans table), enum_ref
    (luaL_ref for enum descriptor), oneof_idx back-pointer.
  * Map fields capture map_key_kind, map_value_kind, and the value's
    sub_plan_idx when the value is a message.
  * Oneofs as a parallel array of {name, member_indices[]}, with
    fields' oneof_idx pointing back to their group.
  * WKT override detection: when desc.encode/desc.decode are set, the
    plan flips has_override=1 and skips field-walk entirely.
  * Extension range hooks captured (proto2 scaffolding for bd-3i).
  * Field-name cache as a Lua table referenced via luaL_ref, so
    encode/decode can do lua_rawgeti instead of re-interning C strings.

Cycle handling: compile_plan stashes the new plan userdata on
desc.c_plan BEFORE recursing into sub-message fields. Person.friends
→ Person resolves to the same userdata; the test asserts identity.

Build entry: 'just build-c' compiles runtime/pb/c/c_runtime.c into
runtime/pb/c_runtime.dylib (or .so on Linux) via the local Makefile.
Module-h location auto-detected the same way bench/c_accel/Makefile
does it. Tarantool's LuaJIT-on-5.1 means we use lua_objlen (not
lua_rawlen) and provide a local abs_idx helper since lua_absindex
isn't available in the 5.1 compat layer.

Smoke test at test/c_runtime_plan_test.lua exercises both codegen
modes (full + runtime). 26 assertions cover:
  - module surface + ABI version + KIND/WIRE constants
  - Address: 4 fields, names, kinds, wire types, tag bytes, optional
  - Person: 14 fields, scalars/enum/message/map/repeated/packed
  - Sub-plan resolution + Person.friends self-reference cycle break
  - Idempotent compile (second call returns cached plan)
  - Result.outcome oneof: 3 members, oneof_idx back-pointers
  - WKT Timestamp: has_override=true, field-walk skipped

Full suite: 771/771 with PB_ENABLE_C=1 (745 existing + 26 new),
745+26 skipped without (silent fallback verified).

Justfile fix: 'just build-c' / 'clean-c' used $(MAKE) which Just
doesn't expand — switched to plain 'make'.

Unblocks bd-y1n (encode scalars), bd-mz6 (decode scalars),
bd-awv (64-bit cdata), bd-rmf (WKT passthrough). bd-mq7 closed.