db: Fix libtool linking against local libstdc++.so

Building the C++ bindings in a484b35b818768487ff27cf06b8c5d4e128126af
introduced this error on systems with /usr/lib/libstdc++.so present:

    /usr/lib/libstdc++.so: file not recognized: File format not recognized

The shipped libtool is sysroot aware, so pass --with-sysroot so it will
extract the sysroot from the compiler.

Signed-off-by: Tyler Hall <tylerwhall@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>