Use RUBY_DEBUG instead of NDEBUG

Assertions in header files slows down an interpreter, so they should be
turned off by default (simple `make`). To enable them, define a macro
`RUBY_DEBUG=1` (e.g. `make cppflags=-DRUBY_DEBUG` or use `#define` at
the very beggining of the file. Note that even if `NDEBUG=1` is defined,
`RUBY_DEBUG=1` enables all assertions.
[Feature #16837]
related: https://github.com/ruby/ruby/pull/3120

`assert()` lines in MRI *.c is not disabled even if `RUBY_DEBUG=0` and
it can be disabled with `NDEBUG=1`. So please consider to use
`RUBY_ASSERT()` if you want to disable them when `RUBY_DEBUG=0`.

include/ruby/internal/core/rmatch.h

@@ -59,7 +59,7 @@ struct RMatch {
     VALUE regexp;  /* RRegexp */
 };
 
-RBIMPL_ATTR_PURE_ON_NDEBUG()
+RBIMPL_ATTR_PURE_UNLESS_DEBUG()
 RBIMPL_ATTR_RETURNS_NONNULL()
 RBIMPL_ATTR_ARTIFICIAL()
 static inline struct re_registers *