Make ivl_alloc.h the last include so it doesn't effect any system includes.

In ivl_alloc.h we redefine malloc(), realloc() and calloc() to have
standard error checking. We don't want to do this for anything that
comes from the standard headers. This specifically doesn't work if
a C++ header files does std::malloc, etc.

Also change to -W instead of -Wextra since that is more portable. I
plan to add a check from -Wextra and use it when available since it
is more descriptive.

vvp/vpi_tasks.cc

@@ -29,11 +29,11 @@
 #ifdef CHECK_WITH_VALGRIND
 # include  "vvp_cleanup.h"
 #endif
-# include  "ivl_alloc.h"
 # include  <cstdio>
 # include  <cstdlib>
 # include  <cstring>
 # include  <cassert>
+# include  "ivl_alloc.h"
 
 static const struct __vpirt vpip_systf_def_rt = {
       vpiUserSystf,