Enable more threads for flto

aclocal.m4

@@ -155,7 +155,7 @@ AC_DEFUN([BIRD_CHECK_LTO],
   bird_tmp_cflags="$CFLAGS"
   bird_tmp_ldflags="$LDFLAGS"
   CFLAGS="-flto"
-  LDFLAGS="-flto"
+  LDFLAGS="-flto=4"
 
   AC_CACHE_CHECK(
     [whether link time optimizer is available],

configure.ac

@@ -156,7 +156,7 @@ fi
 
 if test "$bird_cv_c_lto" = yes; then
   CFLAGS="$CFLAGS -flto"
-  LDFLAGS="$LDFLAGS -flto"
+  LDFLAGS="$LDFLAGS -flto=4"
 fi
 
 AC_MSG_CHECKING([CFLAGS])