Submitted by: Bruce Stephens <bruce.stephens@isode.com>
Use same construct for EXHEADER in srp/Makefile as other makefiles to cope
with possibly empty EXHEADER.
This meant a slight renumbering in util/libeay.num due to symbols
appearing in 1.0.0-stable. However, since there's been no release on
this branch yet, it should be harmless.