correct NINJA_FALLTHROUGH logic

previously, the top option was never used as seemingly intended.
GCC >= 7 has __has_cpp_attribute() and thus much of the original
workarounds aren't needed.

use C++11 [[noreturn]]

only very old compilers wouldn't have this attribute.
the previous code was from 11 years ago.

src/util.h

@@ -26,27 +26,26 @@
 #include <string>
 #include <vector>
 
-#ifdef _MSC_VER
-#define NORETURN __declspec(noreturn)
+#if !defined(__has_cpp_attribute)
+#  define __has_cpp_attribute(x)  0
+#endif
+
+#if __has_cpp_attribute(noreturn)
+#  define NORETURN [[noreturn]]
 #else
-#define NORETURN __attribute__((noreturn))
+#  define NORETURN  // nothing for old compilers
 #endif
 
 /// Log a fatal message and exit.
 NORETURN void Fatal(const char* msg, ...);
 
 // Have a generic fall-through for different versions of C/C++.
-#if defined(__cplusplus) && __cplusplus >= 201703L
-#define NINJA_FALLTHROUGH [[fallthrough]]
-#elif defined(__cplusplus) && __cplusplus >= 201103L && defined(__clang__)
-#define NINJA_FALLTHROUGH [[clang::fallthrough]]
-#elif defined(__cplusplus) && __cplusplus >= 201103L && defined(__GNUC__) && \
-    __GNUC__ >= 7
-#define NINJA_FALLTHROUGH [[gnu::fallthrough]]
-#elif defined(__GNUC__) && __GNUC__ >= 7 // gcc 7
-#define NINJA_FALLTHROUGH __attribute__ ((fallthrough))
-#else // C++11 on gcc 6, and all other cases
-#define NINJA_FALLTHROUGH
+#if __has_cpp_attribute(fallthrough)
+#  define NINJA_FALLTHROUGH [[fallthrough]]
+#elif defined(__clang__)
+#  define NINJA_FALLTHROUGH [[clang::fallthrough]]
+#else
+#  define NINJA_FALLTHROUGH // nothing
 #endif
 
 /// Log a warning message.