diff --git a/configure.ac b/configure.ac
index 118f7f3c..06115532 100644
--- a/configure.ac
+++ b/configure.ac
@@ -125,12 +125,24 @@ AS_IF([test "x$have_gmime_24" = "xno"],[
 AC_SUBST(GMIME_CFLAGS)
 AC_SUBST(GMIME_LIBS)
  
+#
+# we test for gtk, which we need if we want to build 'mug'; for
+# experiments it can try to build with gtk3, but since that is still
+# under development, you need to explicitly enable this with '--with-gtk3'
+#
+AC_ARG_WITH([gtk3],
+   [AS_HELP_STRING([--with-gtk3],
+   [enable support for gtk3])],
+   [],
+   [with_gtk3=no])
+   
+AS_IF([test "x$with_gtk3" != "xno"],[
+  PKG_CHECK_MODULES(GTK,gtk+-3.0,[have_gtk3=yes],[have_gtk3=no])
+  AC_SUBST(GTK_CFLAGS)
+  AC_SUBST(GTK_LIBS)
+])
 
-# do we have gtk3+ installed? this is just for testing the new gtk3
-PKG_CHECK_MODULES(GTK,gtk+-3.0,[have_gtk3=yes],[have_gtk3=no])
-AC_SUBST(GTK_CFLAGS)
-AC_SUBST(GTK_LIBS)
-AS_IF([test "x$have_gtk3" = "xno"],[
+AS_IF([test "x$have_gtk3" != "xyes"],[
   PKG_CHECK_MODULES(GTK,gtk+-2.0,[have_gtk2=yes],[have_gtk2=no])
   AC_SUBST(GTK_CFLAGS)
   AC_SUBST(GTK_LIBS)
@@ -138,6 +150,7 @@ AS_IF([test "x$have_gtk3" = "xno"],[
 AM_CONDITIONAL(HAVE_GTK, [test "x$have_gtk2" = "xyes" -o "x$have_gtk3" = "xyes"])
 
 
+
 # xapian?
 AC_CHECK_PROG(XAPIAN,xapian-config,xapian-config,no)
 AM_CONDITIONAL(HAVE_XAPIAN,test "x$XAPIAN" != "xno")