netsnmp: improve linking avoiding useless -lz listing in shared build

netsnmp: improve linking avoiding useless -lz listing in shared build

In commit:
https://git.buildroot.net/buildroot/commit/?id=13722d58f77d0e9fea9eefc50bf083d19f835433
Patch "0003-configure-Invert-AC_CHECK_LIB-EVP_md5-.-without-lz-w.patch"
was intended to fix AC_CHECK_FUNCS() failure on openssl functions. This
was due to missing -lz during static linking.
But the patch is wrong and results in explicitly linking against -lz in
both shared and static build.
This makes no sense, since shared linking has transitive dependency so
it doesn't need to list -lz after -lssl, -lssl is enough.
Differently static linking needs -lz to be listed after -lssl.

So the real cause of previous build failure:
http://autobuild.buildroot.net/results/881/881139fb049738b16609d39ad5a49bd77ff6b4aa/
is that when AC_CHECK_FUNCS(), $LIBS variable is overwritten with
$LIBCRYPTO without taking into accout previous $LIBS content(i.e. where
-lz is present). This results in AC_CHEC_FUNCS() to fail while trying to
statically link without listing -lz.

Then:
- Remove current "0003-configure-Invert-AC_CHECK_LIB-EVP_md5-.-without-lz-w.patch"
- Add patch "0003-configure-fix-AC_CHECK_FUNCS-EVP_sha224-EVP_sha384-..patch"
  where add $LIBS content to tail of new $LIBS variable like this:
  LIBS="$LIBCRYPTO $LIBS"
  NOTE: $LIBS is at the end to ensure static linking to work correctly.
- Add patch 0004-configure-fix-AC_CHECK_FUNCS-TLS_method-TLSv1_method.patch
  where add $LIBS content to tail of new $LIBS variable like this:
  LIBS="-lssl $LIBCRYPTO $LIBS"
  NOTE: $LIBS is at the end to ensure static linking to work correctly.

This way AC_CHECK_FUNCS(), when static linking, try to link with -lz too
appending it at the end of linking library list.
And after every AC_CHECK_FUNCS(), previously saved $LIBS variable gets
back to its original value(i.e. containing -lz if present) resulting in
having or not -lz appended to library list according to static or
shared build.

Signed-off-by: Giulio Benetti <giulio.benetti@micronovasrl.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>