Don't use Clang nullability qualifiers in strict ISO C mode

`_Nullable` and `_Nonnull` are Clang extensions, so compiling the rbs
headers in a strict ISO C dialect emits `-Wnullability-extension`, which
`-pedantic-errors` turns into an error:

    include/rbs/ast.h:166:44: error: type nullability specifier '_Nonnull' is a Clang extension [-Werror,-Wnullability-extension]
      166 | void rbs_node_list_append(rbs_node_list_t *RBS_NONNULL list, rbs_node_t *RBS_NONNULL node);
          |                                            ^
    include/rbs/defines.h:93:21: note: expanded from macro 'RBS_NONNULL'
       93 | #define RBS_NONNULL _Nonnull

This breaks embedders that build the extension with a conforming dialect.
CRuby's `omnibus compilations` CI job compiles the bundled gems with
`-std=c99 -Werror=pedantic -pedantic-errors` (and c11/c17/c2x), so
building `ext/rbs_extension` there fails once rbs is bundled.

Expand the macros to nothing when `__STRICT_ANSI__` is defined, which is
exactly the case where the qualifiers are not accepted. Non-strict builds,
including Clang's default `-std=gnu*` dialects, keep the annotations, so
nothing is lost for the static analyzer.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Vqx4FeruSY2rScP3frptqZ

Author: claude

include/rbs/defines.h

@@ -87,8 +87,13 @@
  * Nullability annotations for pointer types.
  * Clang supports _Nullable and _Nonnull to indicate whether a pointer may be NULL.
  * On other compilers, these expand to nothing.
+ *
+ * They are Clang extensions, so a strict ISO C compilation rejects them with
+ * `-Wnullability-extension`, which is an error under `-pedantic-errors`. Expand
+ * them to nothing in that case, so that embedders compiling these headers with a
+ * conforming dialect (`-std=c99` and friends define `__STRICT_ANSI__`) still build.
  */
-#ifdef __clang__
+#if defined(__clang__) && !defined(__STRICT_ANSI__)
 #define RBS_NULLABLE _Nullable
 #define RBS_NONNULL _Nonnull
 #else