Correctly spec rb_str_new_cstr as 1.9-only CAPI

Currently, rb_str_new_cstr is spec'ed as a function available under both 1.8
CAPI and 1.9 CAPI in spec/ruby/optional/capi/ext/rubyspec.h. This is not true.
As a version guard is correctly used in spec/ruby/optional/capi/string_spec.rb,
it is only declared under 1.9 CAPI, not under 1.8 CAPI.

In 1.8 mode, rb_str_new_cstr specs are only compiled, not run, thanks to the
version guard. So, there is no spec failures. However, this commit suppresses
the following warning from clang:

  spec/ruby/optional/capi/ext/string_spec.c:165:12: warning:
    implicit declaration of function 'rb_str_new_cstr' is invalid in C99
      [-Wimplicit-function-declaration]
      return rb_str_new_cstr(NULL);
             ^
  1 warning generated.

Also, #undefs aren't needed in spec/ruby/optional/capi/ext/mri.h, because
rb_str_new_cstr is now correctly declared only in 1.9 CAPI.

spec/ruby/optional/capi/ext/mri.h

@@ -18,12 +18,10 @@
 #undef HAVE_RBIGNUM_POSITIVE_P
 #undef HAVE_RBIGNUM_NEGATIVE_P
 #undef HAVE_RB_STR_SET_LEN
-#undef HAVE_RB_STR_NEW_CSTR
 #endif
 
 #ifdef RUBY_VERSION_IS_1_8_EX_1_9
 #undef HAVE_RB_THREAD_BLOCKING_REGION
-#undef HAVE_RB_STR_NEW_CSTR
 #undef HAVE_RB_RATIONAL
 #undef HAVE_RB_RATIONAL1
 #undef HAVE_RB_RATIONAL2

spec/ruby/optional/capi/ext/rubyspec.h

@@ -199,6 +199,7 @@
 #define HAVE_RB_ENC_TO_INDEX               1
 #define HAVE_RB_OBJ_ENCODING               1
 
+#define HAVE_RB_STR_NEW_CSTR               1
 #define HAVE_RB_USASCII_STR_NEW            1
 #define HAVE_RB_USASCII_STR_NEW_CSTR       1
 #define HAVE_RB_EXTERNAL_STR_NEW           1
@@ -432,7 +433,6 @@
 #define HAVE_RB_STR_INTERN                 1
 #define HAVE_RB_STR_LEN                    1
 #define HAVE_RB_STR_NEW                    1
-#define HAVE_RB_STR_NEW_CSTR               1
 #define HAVE_RB_STR_NEW2                   1
 #define HAVE_RB_STR_NEW3                   1
 #define HAVE_RB_STR_NEW4                   1