easylogging++: add emscripten support

This is upstream, but seems to have got lost in conflicts
when merging support for BSDs or android.

external/easylogging++/easylogging++.h

@@ -125,13 +125,16 @@
 #else
 #  define ELPP_OS_NETBSD 0
 #endif
+#if defined(__EMSCRIPTEN__)
+#  define ELPP_OS_EMSCRIPTEN 1
+#endif
 #if (defined(__DragonFly__))
 #   define ELPP_OS_DRAGONFLY 1
 #else
 #   define ELPP_OS_DRAGONFLY 0
 #endif
 // Unix
-#if ((ELPP_OS_LINUX || ELPP_OS_MAC || ELPP_OS_FREEBSD || ELPP_OS_NETBSD || ELPP_OS_SOLARIS || ELPP_OS_AIX || ELPP_OS_DRAGONFLY || ELPP_OS_OPENBSD) && (!ELPP_OS_WINDOWS))
+#if ((ELPP_OS_LINUX || ELPP_OS_MAC || ELPP_OS_FREEBSD || ELPP_OS_NETBSD || ELPP_OS_SOLARIS || ELPP_OS_AIX || ELPP_OS_EMSCRIPTEN || ELPP_OS_DRAGONFLY || ELPP_OS_OPENBSD) && (!ELPP_OS_WINDOWS))
 #  define ELPP_OS_UNIX 1
 #else
 #  define ELPP_OS_UNIX 0
@@ -216,7 +219,7 @@ ELPP_INTERNAL_DEBUGGING_OUT_INFO << ELPP_INTERNAL_DEBUGGING_MSG(internalInfoStre
 #  define ELPP_INTERNAL_INFO(lvl, msg)
 #endif  // (defined(ELPP_DEBUG_INFO))
 #if (defined(ELPP_FEATURE_ALL)) || (defined(ELPP_FEATURE_CRASH_LOG))
-#  if (ELPP_COMPILER_GCC && !ELPP_MINGW && !ELPP_OS_OPENBSD && !ELPP_OS_NETBSD && !ELPP_OS_ANDROID)
+#  if (ELPP_COMPILER_GCC && !ELPP_MINGW && !ELPP_OS_OPENBSD && !ELPP_OS_NETBSD && !ELPP_OS_ANDROID && !ELPP_OS_EMSCRIPTEN)
 #    define ELPP_STACKTRACE 1
 #  else
 #    define ELPP_STACKTRACE 0