diff --git a/lib/libthr/Makefile b/lib/libthr/Makefile
--- a/lib/libthr/Makefile
+++ b/lib/libthr/Makefile
@@ -51,8 +51,10 @@
 
 MAN=	libthr.3
 
+.if ${MK_PTHREADS_INVARIANTS} != "no"
 # enable extra internal consistency checks
 CFLAGS+=-D_PTHREADS_INVARIANTS
+.endif
 
 PRECIOUSLIB=
 
diff --git a/lib/libthr/thread/thr_mutex.c b/lib/libthr/thread/thr_mutex.c
--- a/lib/libthr/thread/thr_mutex.c
+++ b/lib/libthr/thread/thr_mutex.c
@@ -120,7 +120,7 @@
 __weak_reference(_pthread_mutex_isowned_np, pthread_mutex_isowned_np);
 
 static void
-mutex_init_link(struct pthread_mutex *m)
+mutex_init_link(struct pthread_mutex *m __unused)
 {
 
 #if defined(_PTHREADS_INVARIANTS)
diff --git a/share/man/man5/src.conf.5 b/share/man/man5/src.conf.5
--- a/share/man/man5/src.conf.5
+++ b/share/man/man5/src.conf.5
@@ -1463,6 +1463,10 @@
 .Xr gprof 8 .
 This option is deprecated and may not be present in a future version of
 .Fx .
+.It Va WITHOUT_PTHREADS_INVARIANTS
+Disable
+.Xr pthread 3
+extra internal consistency checks.
 .It Va WITHOUT_QUOTAS
 Do not build
 .Xr quota 1
diff --git a/share/mk/src.opts.mk b/share/mk/src.opts.mk
--- a/share/mk/src.opts.mk
+++ b/share/mk/src.opts.mk
@@ -159,6 +159,7 @@
     PKGBOOTSTRAP \
     PMC \
     PPP \
+    PTHREADS_INVARIANTS \
     QUOTAS \
     RADIUS_SUPPORT \
     RBOOTD \