Rewrite the check for libdpdk

Rewrite the check for libdpdk

This fixes various issues, like us not picking up the right include path
and testing for the library in the wrong location.

We now allow you to override the CPPFLAGS and LDFLAGS by doing
./configure LIBDPDK_CPPFLAGS="-I/path/to/your/libdpdk_headers" LIBDPDK_LDFLAGS="-L/path/to/your/libdpdk.so -Wl,--no-as-needed"

The nice thing is that these settings are now logged and remembered
between configure runs.