buildsys: improve cross-build for freebsd and openbsd

on FreeBSD and OpenBSD, the most 3rd-party libs are installed in /usr/local/lib directory, we need to manully add -L/usr/local/lib to LDFLAGS only when do native build.

Signed-off-by: leleliu008 <leleliu008@gmail.com>

configure.ac

@@ -45,8 +45,10 @@ case $host in
         fi
         ;;
     *freebsd*|*openbsd*)
-        # https://github.com/universal-ctags/ctags/issues/3338
-        export LDFLAGS="$LDFLAGS -L/usr/local/lib"
+        if test "$cross_compiling" = "no" ; then
+            # https://github.com/universal-ctags/ctags/issues/3338
+            export LDFLAGS="$LDFLAGS -L/usr/local/lib"
+        fi
         ;;
 esac