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

[soutaro]

_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

CRuby's omnibus compilations, #5 job builds with -std=c99 -Werror=pedantic -pedantic-errors (and c11/c17/c2x), so ext/rbs_extension fails to compile there once rbs 4.1.x is bundled — see ruby/ruby#18274. rbs 4.0.3 had no nullability macros, so this is new in 4.1.x.

Expand the macros to nothing when __STRICT_ANSI__ is defined, which is exactly the case where the qualifiers are rejected. Feature-test macros don't work here: both __has_feature(nullability) and __has_extension(nullability) report 1 even under -std=c99, since the extension is available — it just warns.

Clang's default -std=gnu* dialects keep the annotations, so nothing is lost for the static analyzer. Verified with clang 18: all of src/ compiles cleanly under c99/c11/c17/c2x with -pedantic-errors, and -std=gnu17 still expands to _Nonnull / _Nullable.