problem with missing link to libX11

Hi Gnome developers,

I recently came across a strange compilation problem where multiple
projects fail to compile with the same error? They compiled earlier, and
I have a fresh installation. The error is:
  /usr/bin/ld: ...: undefined reference to symbol '...'
  /usr/bin/ld: note: '...' is defined in DSO .../libX11.so so try adding
it to the linker command line
  .../libX11.so: could not read symbols: Invalid operation

The problem can be fixed by adding $(X_LIBS) to the *_LDADD variable of
Makefile.am, see bug #638510:
 https://bugzilla.gnome.org/show_bug.cgi?id=638510

Projects with this problem are (respective branch):
  gnome-applets (master)
  gnome-panel (gtk3)
  libwnck (gtk3)

I am using Ubuntu 11.04 (development branch).

Any idea?

Cheers,
Enrico