sim: move -Werror disabling to Makefile

For the ports that still don't build with -Werror, rather than disable
the flag at configure time, do it at make time.  This will allow us to
unify these tests in the common sim configure script.

sim/example-synacor/configure

@@ -11149,15 +11149,14 @@ if test "${enable_werror+set}" = set; then :
 fi
 
 
-# Enable -Werror by default when using gcc
 if test "${GCC}" = yes -a -z "${ERROR_ON_WARNING}" ; then
-    ERROR_ON_WARNING=yes
+  ERROR_ON_WARNING=yes
 fi
 
 WERROR_CFLAGS=""
-  if test "${ERROR_ON_WARNING}" = yes ; then
-    WERROR_CFLAGS="-Werror"
-  fi
+if test "${ERROR_ON_WARNING}" = yes ; then
+  WERROR_CFLAGS="-Werror"
+fi
 
 build_warnings="-Wall -Wdeclaration-after-statement -Wpointer-arith \
 -Wpointer-sign \