sim: use -Werror when probing for supported warning flags

When building with clang, we get:

    error: unknown warning option '-Wmissing-parameter-type' [-Werror,-Wunknown-warning-option]

This is because clang only warns by default when encountering an unknown
warning option, and the probe for supported warning flags is done
without -Werror.  All flags are therefore accepted by configure, but
then it breaks when actually compiling a source file with -Werror.

This is equivalent to this commit in gdb:

    3e019bdc20
    gdb: Use -Werror when checking for (un)supported warning flags

We then see some other compilation errors when building with clang and
-Werror, they can be dealt with later.

sim/example-synacor/ChangeLog

@@ -1,3 +1,7 @@
+2021-04-21  Simon Marchi  <simon.marchi@polymtl.ca>
+
+	* configure: Regenerate.
+
 2021-04-18  Mike Frysinger  <vapier@gentoo.org>
 
 	* configure: Regenerate.

sim/example-synacor/configure

@@ -12073,7 +12073,7 @@ $as_echo_n "checking compiler warning flags... " >&6; }
 	-Werr*) WERROR_CFLAGS=-Werror ;;
 	*) # Check that GCC accepts it
 	    saved_CFLAGS="$CFLAGS"
-	    CFLAGS="$CFLAGS $w"
+	    CFLAGS="$CFLAGS -Werror $w"
 	    cat confdefs.h - <<_ACEOF >conftest.$ac_ext
 /* end confdefs.h.  */