libthr: Add src.conf variable WITHOUT_PTHREADS_ASSERTIONS

This patch fixes a bug which prevents building libthr without
_PTHREADS_INVARIANTS defined. The default remains to build libthr
with -D_PTHREADS_INVARIANTS. However, with this patch, if one builds
libthr with WITHOUT_PTHREADS_ASSERTIONS=true then the latency to
acquire+release a default pthread mutex is reduced by roughly 5%, and a
robust mutex by roughly 18% (as measured by a simple synthetic test on a
Xeon E5-2697a based machine).

Reviewed by:	jhb, kib, mjg
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D40900

lib/libthr/Makefile

@@ -51,8 +51,10 @@ SYMBOL_MAPS=${.CURDIR}/pthread.map
 
 MAN=	libthr.3
 
+.if ${MK_PTHREADS_ASSERTIONS} != "no"
 # enable extra internal consistency checks
 CFLAGS+=-D_PTHREADS_INVARIANTS
+.endif
 
 PRECIOUSLIB=
 

lib/libthr/thread/thr_mutex.c

@@ -120,7 +120,7 @@ __weak_reference(_pthread_mutex_getyieldloops_np, pthread_mutex_getyieldloops_np
 __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)

share/mk/src.opts.mk

@@ -159,6 +159,7 @@ __DEFAULT_YES_OPTIONS = \
     PKGBOOTSTRAP \
     PMC \
     PPP \
+    PTHREADS_ASSERTIONS \
     QUOTAS \
     RADIUS_SUPPORT \
     RBOOTD \

tools/build/options/WITHOUT_PTHREADS_ASSERTIONS

@@ -0,0 +1 @@
+Disable debugging assertions in pthreads library.