build: don't fail if libnl-3 is not found

Commit 9298bfbc changed configure to split the libnl into two
separate pkg config checks instead of nesting the second check
on the failure path of the first.  But the default pkg config
behavior is to abort configure if a check fails.  Since we have
a second check lined up, we need an explicit failure case that
does not abort if the first check fails.

Meanwhile, commit 51b708c6 is reverted.  It did not fix any
behavior, and in fact, introduced a regression to the fallback
case when the user explicitly sets $LIBNL_CFLAGS.

* configure.ac: Don't abort if libnl-3 is not found.

configure.ac

@@ -2915,7 +2915,7 @@ if test "$with_linux" = "yes"; then
         fi
     done
     case $libnl_ldd:${LIBNL_CFLAGS+set} in
-        *libnl.so.1*) ;;
+        *libnl.so.1*:) ;;
         *)
         PKG_CHECK_MODULES([LIBNL], [libnl-3.0], [
             have_libnl=yes
@@ -2924,7 +2924,7 @@ if test "$with_linux" = "yes"; then
             PKG_CHECK_MODULES([LIBNL_ROUTE3], [libnl-route-3.0])
             LIBNL_CFLAGS="$LIBNL_CFLAGS $LIBNL_ROUTE3_CFLAGS"
             LIBNL_LIBS="$LIBNL_LIBS $LIBNL_ROUTE3_LIBS"
-        ], []) ;;
+        ], [:]) ;;
     esac
     if test "$have_libnl" = no; then
         PKG_CHECK_MODULES([LIBNL], [libnl-1 >= $LIBNL_REQUIRED], [