Option gets used for C as well leading to gcc warnings and a hard
error with clang. Option isn't actually needed, so just remove it.

https://bugs.gentoo.org/830571
--- a/premake4.lua
+++ b/premake4.lua
@@ -47,3 +47,2 @@
 	  defines { "PHYSFS_SUPPORTS_ZIP" }
-	  buildoptions { "-std=gnu++0x" }