PlayerControl: work around std::nanf() not being available on MacPorts

src/PlayerControl.cxx

@@ -44,7 +44,13 @@ player_control::player_control(unsigned _buffer_chunks,
 	 next_song(nullptr),
 	 cross_fade_seconds(0),
 	 mixramp_db(0),
+#if defined(__GLIBCXX__) && !defined(_GLIBCXX_USE_C99_MATH_TR1)
+	 /* workaround: on MacPorts, this option is disabled on gcc47,
+	    and therefore std::nanf() is not available */
+	 mixramp_delay_seconds(nanf("")),
+#else
 	 mixramp_delay_seconds(std::nanf("")),
+#endif
 	 total_play_time(0),
 	 border_pause(false)
 {