C Extension: Use #if instead of #ifdef for JSON_DEBUG

[samyron]

While profiling the C extension, I kept hitting code within the #ifdef JSON_DEBUG. Using #ifdef checks if the identifier is defined, not the value.

I did not have the JSON_DEBUG environment variable set but noticed the following in json.h:

#ifdef RUBY_DEBUG
#ifndef JSON_DEBUG
#define JSON_DEBUG RUBY_DEBUG
#endif
#endif

Adding #error "RUBY_DEBUG defined but not JSON_DEBUG" after the #ifndef and running rake outputs the following:

In file included from ../../../../../../ext/json/ext/generator/generator.c:1:
../../../../../../ext/json/ext/generator/../json.h:82:2: error: "RUBY_DEBUG defined but not JSON_DEBUG"
   82 | #error "RUBY_DEBUG defined but not JSON_DEBUG"
      |  ^
1 error generated.

I should note that I am not running debug build of Ruby. Adding a printf("RUBY_DEBUG=%lu\n", RUBY_DEBUG); outputs a lot of RUBY_DEBUG=0.

My assumption is we want this code to be excluded if JSON_DEBUG is 0.

[samyron]

This is optional but it allows JSON_DEBUG=0 to not trigger debugging code. Feel free to remove if this isn't needed.