ci: static link AppVeyor binaries

Author: JaCzekanski

premake5.lua

@@ -133,10 +133,15 @@ filter "configurations:Release"
 	optimize "Full"
 
 filter {"configurations:Release"}
-	if os.getenv("CI") == true then
+	if os.getenv("CI") ~= nil then
 		flags { "LinkTimeOptimization" }
 	end
 
+filter {"configurations:Release", "system:linux"}
+	if os.getenv("CI") ~= nil then
+		linkoptions { "-static-libstdc++" }
+	end
+
 include "premake/chdr.lua"
 include "premake/flac.lua"
 include "premake/glad.lua"