diff --git a/Makefile.inc b/Makefile.inc
index 2ae1b810..e5390ced 100644
--- a/Makefile.inc
+++ b/Makefile.inc
@@ -14,11 +14,22 @@ IDIRS	:=	${IDIRS}
 # Default is no man pages
 MAN	=
 
+# The SUBDIR_DEPTH variable (defined above) initializes tracking the
+# subdirectory depth, but it's not fully set at this stage in the build
+# process.  Instead, we jump back to the shell and print the (finalized) value
+# of SUBDIR_DEPTH.  This avoids an infinite loop because we query the Makefile
+# (which does not include ../Makefile.inc), rather than Makefile.BSD (which
+# would end up including this file again).
+FINALIZED_SUBDIR_DEPTH	!=	${MAKE} -v SUBDIR_DEPTH
+
 .if !defined(NOLIBALL)
 # Link everything to liball.a, unless they specifically ask not to use it.
-# If appropriate, metabuild.sh will do:
+LIBALL	=	${SUBDIR_DEPTH}/liball/liball.a
+.if exists(${FINALIZED_SUBDIR_DEPTH}/liball/optional_mutex_normal)
+# If we have optional_mutex, add it. If appropriate, metabuild.sh will do:
 #     s/optional_mutex_normal/optional_mutex_pthread/g
-LIBALL	=	${SUBDIR_DEPTH}/liball/liball.a ${SUBDIR_DEPTH}/liball/optional_mutex_normal/liball_optional_mutex_normal.a
+LIBALL	+=	${SUBDIR_DEPTH}/liball/optional_mutex_normal/liball_optional_mutex_normal.a
+.endif
 LDADD	+=	${LIBALL}
 DPADD	+=	${LIBALL}
 .endif