res_rtp_asterisk: Instead of ./configure use OPENSSL_NO_SRTP.

Previously, Asterisk used its script ./configure, to test whether OpenSSL was
built with no-srtp (or was simply too old). However, the header file
<openssl/opensslconf.h> is the preferred way to detect the local configuration
of OpenSSL.

As a positive side-effect the script ./configure does not interleave the
detection of the Open Settlement Protocol Toolkit (OSPTK) with the detection of
individual features of OpenSSL anymore.

Change-Id: I3c77c7b00b2ffa2e935632097fa057b9fdf480c0