Fix detection of DBD modules for multiarch setups.

On Debian unstable, the DBD modules are now under
/usr/lib/${MULTIARCH}/dbd (where MULTIARCH depends on the architecture). This
patch ensures that the DBD modules are correctly detected in that case, using
pkg-config.

configure.ac

@@ -604,7 +604,7 @@ then
       [[GNC_DBD_DIR="$with_dbi_dbd_dir"
        EXTRA_SEARCH_LIBS=$GNC_DBD_DIR]],
       [[GNC_DBD_DIR=""
-       EXTRA_SEARCH_LIBS="/usr/lib/dbd:/usr/lib64/dbd:${libdir}/dbd"]]
+       EXTRA_SEARCH_LIBS="/usr/lib/dbd:/usr/lib64/dbd:${libdir}/dbd:$(pkg-config --variable=libdir dbi)/dbd"]]
        )
     case $host_os in
        win32*|mingw*)