build: Various OpenBSD issues

OpenBSD's 'find' doesn't take the -delete argument so you have to pipe
through 'xargs rm -rf'.

'echo -e' doesn't like \t starting a line. It just prints 't' which
causes the libasteriskpj.exports file to be garbage.  They were just
cosmetic so they were removed.

librt doesn't exist so the link of libasteriskpj.so fails. It's not
actually needed for linux anyway so -lrt was removed from the link.

res_rtp_asterisk was failing to load because of an undefined
DTLS_method. '|| defined(LIBRESSL_VERSION_NUMBER)' was added to the #if
so DTLSv1_method is used instead.

ASTERISK-26608

Change-Id: I926ec95b0b69633231e3ad1d6e803b977272c49c