Turn on -Werror and -Wmissing-prototypes

When compiling with GCC turn on -Wmissing-prototypes, and by default
turn on -Werror as well.

As with most gnome modules:

 --enable-compile-warnings=minimum/maximum

Can be used to disable -Werror (they are the same)

http://bugzilla.gnome.org/show_bug.cgi?id=590998

configure.ac

@@ -80,17 +80,31 @@ AC_SUBST(GIRDIR)
 TYPELIBDIR="$($PKG_CONFIG --variable=typelibdir gobject-introspection-1.0)"
 AC_SUBST(TYPELIBDIR)
 
+# Stay command-line compatible with the gnome-common configure option. Here
+# minimum/yes/maximum are the same, however.
+AC_ARG_ENABLE(compile_warnings,
+  AC_HELP_STRING([--enable-compile-warnings=@<:@no/minimum/yes/maximum/error@:>@],
+                 [Turn on compiler warnings]),,
+  enable_compile_warnings=error)
+
 changequote(,)dnl
-if test "x$GCC" = "xyes"; then
+if test "$enable_compile_warnings" != no ; then
+  if test "x$GCC" = "xyes"; then
     case " $CFLAGS " in
     *[\ \	]-Wall[\ \	]*) ;;
     *) CFLAGS="$CFLAGS -Wall" ;;
     esac
-
-  case " $OBJCFLAGS " in
-  *[\ \	]-Wall[\ \	]*) ;;
-  *) OBJCFLAGS="$OBJCFLAGS -Wall" ;;
+    case " $CFLAGS " in
+    *[\ \	]-Wmissing-prototypes[\ \	]*) ;;
+    *) CFLAGS="$CFLAGS -Wmissing-prototypes" ;;
     esac
+    if test "$enable_compile_warnings" = error ; then
+      case " $CFLAGS " in
+      *[\ \	]-Werror[\ \	]*) ;;
+      *) CFLAGS="$CFLAGS -Werror" ;;
+      esac
+    fi
+  fi
 fi
 changequote([,])dnl