Avoid like in Makefile.rules
ld: -rpath can only be used when targeting Mac OS X 10.5 or later

--- unittests/Makefile.unittest
+++ unittests/Makefile.unittest
@@ -37,7 +37,9 @@
 ifeq ($(ENABLE_SHARED), 1)
   # Add the absolute path to the dynamic library.  This is ok because
   # we'll never install unittests.
+ifneq ($(DARWIN_MAJVERS),4)
   LD.Flags += $(RPATH) -Wl,$(SharedLibDir)
+endif
   # Also set {DYLD,LD}_LIBRARY_PATH because OSX ignores the rpath most
   # of the time.
   Run.Shared := $(SHLIBPATH_VAR)="$(SharedLibDir)$${$(SHLIBPATH_VAR):+:}$$$(SHLIBPATH_VAR)"