Fix the GCC build (#40)

* Make it compile with GCC.  
* Add GCC to CI.
* Add warning to documentation about using GCC.

src/mem/largealloc.h

@@ -8,6 +8,8 @@
 #include "baseslab.h"
 #include "sizeclass.h"
 
+#include <new>
+
 namespace snmalloc
 {
   template<class PAL>