Add automake support for --enable-gcc-hidden-visibility.

configure.in.in

@@ -1,4 +1,5 @@
 #MIN_CONFIG(3.2)
+KDE_ENABLE_HIDDEN_VISIBILITY
 CXXFLAGS="$CXXFLAGS $KDE_DEFAULT_CXXFLAGS"
 
 if test "$build_arts" = "yes"; then