Check for dmalloc at the end of the configure

Check for dmalloc at the end of the configure

Enabling dmalloc in Samba3 build leads to the wrong detection of the
strndup() function - there isn't one in FreeBSD prior to 7.2, but
dmalloc defines it, so, farther tests with -ldmalloc added wrongly
find it. the cheapest fix is to move dmalloc detection and inclusion
to the bottom of configure, so it can't affect detection of the system
capabilities.

Here is the patch.

With regards,
Timur Bakeyev.

Signed-off-by: Stefan Metzmacher <metze@samba.org>