Disable warnings about deprecated declarations in wxGTK3 build

There are just too many of these warnings because GTK+ people are extremely
deprecation-happy and like marking functions which need to be used for the
code to work with the older GTK+ versions as deprecated. Because of this, in
many cases we have no choice but to continue to use the now deprecated
functions and the only way to avoid them is to pepper the code with the
pragmas doing this. Disabling the warning globally seems like the lesser evil
because not disabling them at all is worse than useless as the flood of the
unavoidable deprecation warnings hides any real ones that might occur.

configure

@@ -36039,7 +36039,7 @@ case ".$ac_cv_cxxflags_gcc_option__Woverloaded_virtual" in
 esac
 
 
-                    if test "$wxUSE_MAC" = 1 ; then
+                        if test "$WXGTK3" = 1 -o "$wxUSE_MAC" = 1 ; then
         CXXWARNINGS="$CXXWARNINGS -Wno-deprecated-declarations"
     fi
 fi

configure.in

@@ -7497,11 +7497,12 @@ elif test "$GXX" = yes ; then
     CXXWARNINGS="-Wall -Wundef -Wunused-parameter -Wno-ctor-dtor-privacy"
     AX_CXXFLAGS_GCC_OPTION(-Woverloaded-virtual, CXXWARNINGS)
 
-    dnl when building under Mac we currently get hundreds of deprecation
-    dnl warnings for Cocoa symbols from the standard headers -- disable them
-    dnl as we already know that they're deprecated and nothing else can be seen
-    dnl with these warnings on
-    if test "$wxUSE_MAC" = 1 ; then
+    dnl when building using GTK+ 3 or Cocoa we currently get tons of deprecation
+    dnl warnings from the standard headers -- disable them as we already know
+    dnl that they're deprecated but we still have to use them to support older
+    dnl toolkit versions and leaving this warning enabled prevents seeing any
+    dnl other ones
+    if test "$WXGTK3" = 1 -o "$wxUSE_MAC" = 1 ; then
         CXXWARNINGS="$CXXWARNINGS -Wno-deprecated-declarations"
     fi
 fi