With the current vala master (last commit=8ec9c6e175ce7f08bc63b133fc264420fb35c7c1), if you try to compile caribou you get this error:

libxml-2.0.vapi:340.2-340.20: error: `Xml' already contains a definition for `Buffer'
	public class Buffer {
	^^^^^^^^^^^^^^^^^^^
libxml-2.0.vapi:327.2-327.20: note: previous definition of `Buffer' was here
	public class Buffer {
	^^^^^^^^^^^^^^^^^^^
Compilation failed: 1 error(s), 0 warning(s)


Related with having a libxml symbol duplicate. Anyway, I made a git bisect to see when this starts, and it seems that there are another duplicate symbols, as the first commit causing problems with caribou is:

eac286586f6c11fa29f366cda702749fd010c50e is the first bad commit
commit eac286586f6c11fa29f366cda702749fd010c50e
Author: Evan Nemerson <evan@coeus-group.com>
Date:   Fri Feb 8 00:21:39 2013 -0800

    x11: add Display.list_properties and XA_STRING
    
    Fixes bug 690905.


That causes the next error while compiling caribou:

x11.vapi:1265.2-1265.31: error: `X' already contains a definition for `XA_STRING'
	public const X.Atom XA_STRING;
	^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
x11.vapi:1249.2-1249.31: note: previous definition of `XA_STRING' was here
	public const X.Atom XA_STRING;
	^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Compilation failed: 1 error(s), 0 warning(s)