strsep: check for availability (Solaris support)

Solaris does not ship a version of the strsep() function.  This change
adds a check to "configure" to see whether notmuch needs to provide its
own implementation, and if so, it uses the new version in
"compat/strsep.c" (which was copied from Mutt, and apparently before
that from glibc).

Signed-off-by: Vladimir Marek <vlmarek@volny.cz>