Improving here means do not set LIBS/CFLAGS if library is not detected.
This prevents some unknown options based to ld when host is Linux and
target is mingw32/64.  Further improvements are needed if one wants this
snmp support when cross compiling to really work.

Also, to make commit bisectable, updated all autofoo files using
'autoreconf -i -f' and repatching ltmain.sh.

Also, test-driver was missing from git repo but the Makefile.in
checked in expected it to exist.