gtk_clipboard_get_selection() was added in gtk+ 3.21.1.
Gtk::Clipboard::get_selection() was added in gtkmm 3.21.2.
That explains why it's no problem when building 3.20.1.

I don't understand why it's a problem when you build any version of gtkmm.
Do you link against a new enough version of gtk+?

Yes, I am linking against gtk+ 3.22.7.

When I build gtk+ and gtkmm with gcc and g++, it does not add a leading
underscore to the function names in the .o and .so files. The undefined symbol
in your case is _gtk_clipboard_get_selection with a leading underscore.
Is it possible that there is a gtk_clipboard_get_selection symbol in the
libgtk-3.so file, but gtkmm's clipboard.o file requires
_gtk_clipboard_get_selection? I admit that it does not seem probable. If there
were this kind of mismatch between the building of gtk+ and the building of
gtkmm, I'd expect hundreds of undefined symbols, not just one.

I did an object dump on libgtk-3.dylib and did not see gtk_clipboard_get_selection. Perhaps that is the problem? 

Here is the result of my object dump:

Parthas-MacBook-Pro:gtkmm-3.22.0 partha$ objdump -t ~/local10.9/lib/libgtk-3.dylib |grep -i gtk_clipboard_get
00000000005d2f70 l       __DATA,__const	_gtk_clipboard_get_type.clipboard_info
00000000005db2b8 l       __DATA,__bss	_gtk_clipboard_get_type.clipboard_type
0000000000366900 g     F __TEXT,__text	_gtk_clipboard_get
0000000000367640 g     F __TEXT,__text	_gtk_clipboard_get_display
0000000000366690 g     F __TEXT,__text	_gtk_clipboard_get_for_display
0000000000366cf0 g     F __TEXT,__text	_gtk_clipboard_get_owner
00000000003665b0 g     F __TEXT,__text	_gtk_clipboard_get_type

In case you wish to know the clang version:

Parthas-MacBook-Pro:gtkmm-3.22.0 partha$ clang -v
Apple LLVM version 8.0.0 (clang-800.0.42.1)
Target: x86_64-apple-darwin16.5.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin

I am running Sierra 10.12.4

I would have expected to find both _gtk_clipboard_get_default and
_gtk_clipboard_get_selection in the object dump. gtk_clipboard_get_default()
has not been wrapped in gtkmm. That explains why it's not reported as a missing
symbol.

Have you built gtk+ from source? Can you check what's in gtkclipboard.c?
gtk_clipboard_get_default() must be declared in gtkclipboard.h, or else your
build would fail during compilation instead of linking.

(In reply to Kjell Ahlstedt from comment #5)
> gtk_clipboard_get_default() must be declared in gtkclipboard.h, or else your
> build would fail during compilation instead of linking.

Should be "gtk_clipboard_get_selection() must be ..."

yes, I build from source obtained from http://ftp.gnome.org/pub/GNOME/sources/

I think this maybe the source of the problem. Note that I am building on a Mac. So, you have 2 gtkclipboard files: gtkclipboard.c gtkclipboard-quartz.c. 

In gtkclipboard.c, you have the correct symbol and you are good:

2222-GdkAtom
2223:gtk_clipboard_get_selection (GtkClipboard *clipboard)
2224-{
2225-  g_return_val_if_fail (GTK_IS_CLIPBOARD (clipboard), GDK_NONE);
2226-
2227-  return clipboard->selection;
2228-}

In gtkclipboard-quartz.c, you don't have this function and hence the issue.

See gtk+ bug 772281

Gtk+ bug 772281 has been fixed in the gtk-3-22 and master branches.
gtk_clipboard_get_selection() is now defined in gtkclipboard-quartz.c.

The fix of gtk+ bug 784323 adds a definition of gtk_clipboard_get_default() to
gtkclipboard-quartz.c, but that's not important for users of gtkmm.
Gtkmm does not wrap gtk_clipboard_get_default(). There's no reason to wrap it.
Gtk::Clipboard::get_for_display() with the default value of its second parameter
does exactly what Gtk::Clipboard::get_default() would do.