command/Error: improve libstdc++ 4.9.x detection for std::rethrow_if_nested() workaround

NEWS

 ver 0.20.6 (not yet released)
 * decoder
   - mpcdec: fix crash (division by zero) after seeking
+* workaround for GCC 4.9.4 / libstdc++ bug (build failure)
 
 ver 0.20.5 (2017/02/20)
 * tags

src/command/CommandError.cxx

@@ -29,6 +29,29 @@
 
 #include <assert.h>
 
+#define GLIBCXX_490 20140422
+#define GLIBCXX_491 20140716
+#define GLIBCXX_492 20141030
+#define GLIBCXX_492_Debian_9 20141220
+#define GLIBCXX_493 20150626
+#define GLIBCXX_494 20160803
+#define GLIBCXX_49X_NDK_r13b 20150123
+
+/* the big mess attempts to detect whether we're compiling with
+   libstdc++ 4.9.x; __GLIBCXX__ is a date tag and cannot be used to
+   check the major version; and just checking the compiler version
+   isn't enough, because somebody could use an old libstdc++ with
+   clang - SIGH! */
+#if GCC_OLDER_THAN(5,0) || (defined(__GLIBCXX__) &&		     \
+	(__GLIBCXX__ == GLIBCXX_490 || __GLIBCXX__ == GLIBCXX_491 || \
+	 __GLIBCXX__ == GLIBCXX_492 || \
+	 __GLIBCXX__ == GLIBCXX_492_Debian_9 || \
+	 __GLIBCXX__ == GLIBCXX_493 || \
+	 __GLIBCXX__ == GLIBCXX_494 || \
+	 __GLIBCXX__ == GLIBCXX_49X_NDK_r13b))
+#define GLIBCXX_49X
+#endif
+
 gcc_const
 static enum ack
 ToAck(PlaylistResult result)
@@ -100,13 +123,13 @@ ToAck(std::exception_ptr ep)
 		return ACK_ERROR_SYSTEM;
 	} catch (const std::invalid_argument &e) {
 		return ACK_ERROR_ARG;
-#if defined(__GLIBCXX__) && __GLIBCXX__ < 20151204
+#ifdef GLIBCXX_49X
 	} catch (const std::exception &e) {
 #else
 	} catch (...) {
 #endif
 		try {
-#if defined(__GLIBCXX__) && __GLIBCXX__ < 20151204
+#ifdef GLIBCXX_49X
 			/* workaround for g++ 4.x: no overload for
 			   rethrow_exception(exception_ptr) */
 			std::rethrow_if_nested(e);