[ruby/bigdecimal] Remove checks for `struct RRational` and `struct RComplex`

These are used to see only if `RRATIONAL` and `RCOMPLEX` are
available, however, these two are macros and can be checked with
`#ifdef` directly.

https://github.com/ruby/bigdecimal/commit/175bbacd43

ext/bigdecimal/extconf.rb

@@ -67,10 +67,8 @@ have_header("ruby/atomic.h")
 have_header("ruby/internal/has/builtin.h")
 have_header("ruby/internal/static_assert.h")
 
-have_type("struct RRational", "ruby.h")
 have_func("rb_rational_num", "ruby.h")
 have_func("rb_rational_den", "ruby.h")
-have_type("struct RComplex", "ruby.h")
 have_func("rb_complex_real", "ruby.h")
 have_func("rb_complex_imag", "ruby.h")
 have_func("rb_array_const_ptr", "ruby.h")

ext/bigdecimal/missing.h

@@ -126,7 +126,7 @@ char *BigDecimal_dtoa(double d_, int mode, int ndigits, int *decpt, int *sign, c
 static inline VALUE
 rb_rational_num(VALUE rat)
 {
-#ifdef HAVE_TYPE_STRUCT_RRATIONAL
+#ifdef RRATIONAL
     return RRATIONAL(rat)->num;
 #else
     return rb_funcall(rat, rb_intern("numerator"), 0);
@@ -138,7 +138,7 @@ rb_rational_num(VALUE rat)
 static inline VALUE
 rb_rational_den(VALUE rat)
 {
-#ifdef HAVE_TYPE_STRUCT_RRATIONAL
+#ifdef RRATIONAL
     return RRATIONAL(rat)->den;
 #else
     return rb_funcall(rat, rb_intern("denominator"), 0);
@@ -152,7 +152,7 @@ rb_rational_den(VALUE rat)
 static inline VALUE
 rb_complex_real(VALUE cmp)
 {
-#ifdef HAVE_TYPE_STRUCT_RCOMPLEX
+#ifdef RCOMPLEX
   return RCOMPLEX(cmp)->real;
 #else
   return rb_funcall(cmp, rb_intern("real"), 0);
@@ -164,7 +164,7 @@ rb_complex_real(VALUE cmp)
 static inline VALUE
 rb_complex_imag(VALUE cmp)
 {
-# ifdef HAVE_TYPE_STRUCT_RCOMPLEX
+# ifdef RCOMPLEX
   return RCOMPLEX(cmp)->imag;
 # else
   return rb_funcall(cmp, rb_intern("imag"), 0);