Add the volatile type qualifier to the global variables

representing external streams.

As we expect these variables to be modifed concurrently or via
interrupts, the volatile qualifier is appropriate, and prevents
the compiler from performing code motion that might cause unexpected
behavior.

Ideally, these reads (which occur at the beginning of the step function)
should also be protected by a critical section that masks interrupts or
takes a mutex.

Partially addresses #257

copilot-c99/src/Copilot/Compile/C99/CodeGen.hs

@@ -31,7 +31,7 @@ genfun name expr ty = C.FunDef cty name [] cvars [C.Return $ Just cexpr] where
 mkextdecln :: External -> C.Decln
 mkextdecln (External name _ ty) = decln where
   decln = C.VarDecln (Just C.Extern) cty name Nothing
-  cty   = transtype ty
+  cty   = C.Volatile (transtype ty)
 
 -- | Make a declaration for a copy of an external variable.
 mkextcpydecln :: External -> C.Decln