In the Makefile there's the following snippet:
$(LN_S) -f `readlink $(DESTDIR)$(libdir)/libp11-kit.so` $(DESTDIR)$(libdir)/p11-kit-proxy.so

However, on Mac OS X the library is named libp11-kit.dylib so in the above command the source of the link resolves to nothing, the destination becomes the source and the link to a non-existent file is created in the working directory.

The following patch fixes the issue:

--- BEGIN PATCH ---
diff --git a/p11-kit/Makefile.am b/p11-kit/Makefile.am
--- a/p11-kit/Makefile.am
+++ b/p11-kit/Makefile.am
@@ -87,7 +87,7 @@
 
 # Proxy module is actually same as library, so install a link
 install-exec-hook:
-	$(LN_S) -f `readlink $(DESTDIR)$(libdir)/libp11-kit.so` $(DESTDIR)$(libdir)/p11-kit-proxy.so
+	$(LN_S) -f `readlink $(DESTDIR)$(libdir)/libp11-kit.{so,dylib}` $(DESTDIR)$(libdir)/p11-kit-proxy.so
 	$(MKDIR_P) $(DESTDIR)$(p11_package_config_modules)
 
 endif
--- END PATCH ---

There is probably a more elegant way to do the above, but unfortunately I'm not an expert on autotools.