Add search for pthreads library if --enable-pthreads.

src/configure.in

@@ -43,6 +43,22 @@ if test "$ok" = "yes"; then
         # should we define DEBUG too?
 fi
 
+dnl pthreads
+AC_ARG_ENABLE(threads, [  --enable-threads          use threading], ok=$enableval, ok=no)
+if test yes = $ok; then
+dnl "aclocal" finds acx_pthread.m4 and includes it, so we can use ACX_PTHREAD here.
+	ACX_PTHREAD([THREADLIBS="$PTHREAD_LIBS"
+		     PTHREAD_CFLAGS="$PTHREAD_CFLAGS"
+		     CC="$PTHREAD_CC"])
+
+	if test -z "$THREADLIBS$PTHREAD_CFLAGS"; then
+		AC_MSG_WARN([couldn't find threads library for --enable-threads])
+	fi
+	AC_DEFINE(HAVE_THREADS, 1, [Define if we have a threads library.])
+fi
+AC_SUBST(THREADLIBS)
+AC_SUBST(PTHREAD_CFLAGS)
+
 
 AC_ARG_WITH(mesa, [  --with-mesa        force the use of Mesa instead of other graphics libs], with_mesa=$withval, with_mesa=maybe)
 
@@ -164,7 +180,7 @@ if test -d "/System/Library"; then
 	# Maybe MacOS X
 
         XLIBS=""
-	AC_SEARCH_LIBS(sqrtf, [mx], AC_DEFINE(HAVE_SQRTF) XLIBS="-lmx")
+	AC_SEARCH_LIBS(sqrtf, [mx], [AC_DEFINE(HAVE_SQRTF) XLIBS="-lmx"])
 
         GLLIBS="-framework AGL -framework OpenGL -framework Carbon -framework ApplicationServices"
 	PV_FLAGS="-Dunix -DHAVE_BUILTIN_ALLOCA"
@@ -259,7 +275,6 @@ AC_CHECK_FUNCS(ftime gethostname gettimeofday getwd mkdir select strdup strtod s
 
 AC_C_BIGENDIAN
 
-
 dnl -- starlab stuff
 
 AC_ARG_WITH(kira, [  --with-kira   force the use of KIRA], with_kira=$withval, with_kira=maybe)