🔨 using __cpp_exceptions to detect exception support #498

I used __EXCEPTIONS to detect whether exceptions are supported.
Apparently, this is a macro that is only used by libstdc++
(https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64276). It’s much cleaner
to use __cpp_exceptions as it is in the standard since C++98.

Note that compiling the unit-tests with “-fno-exceptions” still does
not work, because Catch uses throw internally. However, the library’s
exceptions can be switched off by defining JSON_NOEXCEPTION.

src/json.hpp

@@ -82,7 +82,7 @@ SOFTWARE.
 #endif
 
 // allow to disable exceptions
-#if not defined(JSON_NOEXCEPTION) || defined(__EXCEPTIONS)
+#if __cpp_exceptions && not defined(JSON_NOEXCEPTION)
     #define JSON_THROW(exception) throw exception
     #define JSON_TRY try
     #define JSON_CATCH(exception) catch(exception)

src/json.hpp.re2c

@@ -82,7 +82,7 @@ SOFTWARE.
 #endif
 
 // allow to disable exceptions
-#if not defined(JSON_NOEXCEPTION) || defined(__EXCEPTIONS)
+#if __cpp_exceptions && not defined(JSON_NOEXCEPTION)
     #define JSON_THROW(exception) throw exception
     #define JSON_TRY try
     #define JSON_CATCH(exception) catch(exception)