diff --git a/cad/irsim/Makefile b/cad/irsim/Makefile
index e983d1661b93..c759b144c778 100644
--- a/cad/irsim/Makefile
+++ b/cad/irsim/Makefile
@@ -22,7 +22,8 @@ CONFIGURE_WRKSRC=	${WRKSRC}/scripts/
 
 CFLAGS+=	-I${X11BASE}/include
 LDFLAGS+=	-L${X11BASE}/lib
-CONFIGURE_ENV+=	PIC_CFLAGS="-fpic" LDFLAGS="${LDFLAGS}"
+CONFIGURE_ENV+=	PIC_CFLAGS="-fpic" LDFLAGS="${LDFLAGS}" \
+		PTHREAD_LIBS="${PTHREAD_LIBS}"
 
 OPTIONS=	SCHEME	"Enable built-in Scheme interpreter" on \
 		TCL	"Enable built-in  Tcl interpreter" off
diff --git a/cad/irsim/files/patch-scripts_configure b/cad/irsim/files/patch-scripts_configure
index 658cf0d73b47..e6d86786a1ee 100644
--- a/cad/irsim/files/patch-scripts_configure
+++ b/cad/irsim/files/patch-scripts_configure
@@ -1,5 +1,5 @@
 --- scripts/configure.orig	Tue Oct 24 21:09:50 2006
-+++ scripts/configure	Sun Dec  3 02:20:09 2006
++++ scripts/configure	Wed Jan 10 21:17:08 2007
 @@ -8180,8 +8180,9 @@
  fi
  
@@ -11,6 +11,15 @@
        TCL_LIB_NAME="tcl${TCL_MAJOR_VERSION}${TCL_MINOR_VERSION}"
        TK_LIB_NAME="tk${TK_MAJOR_VERSION}${TK_MINOR_VERSION}"
        ;;
+@@ -8421,7 +8422,7 @@
+ #define HAVE_PTHREADS 1
+ _ACEOF
+ 
+-    gr_libs="$gr_libs -lpthread"
++    gr_libs="$gr_libs $PTHREAD_LIBS"
+     gr_srcs="$gr_srcs thread.c"
+     gr_hsrcs=""
+     gr_hprog=""
 @@ -10258,9 +10259,8 @@
  
      *-freebsd*)