On modern Linux systems (in my case, Fedora 13 and 14), the sem_close and other functions are in the rt and pthread libraries. This combination is not checked for properly by the unix package's autoconf script. As a result attempting to build the unix package will succeed, but linking applications against the misconfigured unix will result in undefined symbol errors for e.g. sem_close.

I believe that the C compiler needs to be invoked with -pthread for the right thing to happen, as unix.buildinfo contains a reference to rt that is nevertheless not enough:

buildable: True
cc-options: 
ld-options: 
extra-libraries:  rt util dl 

Here's my current workaround hack:

--- old-unix/configure.ac	2010-11-23 15:25:33.741849630 -0800
+++ new-unix/configure.ac	2010-11-23 15:25:33.760849987 -0800
@@ -215,6 +215,9 @@
 	PTHREAD_CFLAGS=-pthread
 	PTHREAD_LDFLAGS=-pthread
 	;;
+linux*)
+	PTHREAD_LIBS=pthread
+	;;
 esac
 AC_SUBST(PTHREAD_CFLAGS)
 AC_SUBST(PTHREAD_LDFLAGS)