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

configure.in.in

@@ -3,6 +3,8 @@
 #  can compile without KDE, optionally)
 AM_CONDITIONAL(KDE_INSTALLED, test "$have_kde" = "yes")
 
+KDE_ENABLE_HIDDEN_VISIBILITY
+
 dnl Checks for header files.
 AC_HEADER_DIRENT
 AC_HEADER_STDC