configure.ac: simplify crypto library configuration

configure.ac: simplify crypto library configuration

This reworks the crypto library configuration, to make it both simpler to
understand and more usable:

 * Only check for OpenSSL when building against OpenSSL (and similar for
   PolarSSL/mbed TLS).
 * Bail out early if a problem with the library is detected.
 * Set CRYPTO_{LIBS,FLAGS} immediately after the crypto library checks,
   removing the need for an extra switch-case later on.
 * We no longer support building openvpn with crypto but without ssl, so
   we can also simplify the logic in configure.ac accordingly.

As a 'side effect' (this actually triggered me), this fixes a bug that
would cause a user-specified OPENSSL_{CRYPTO,SSL}_LIBS to be overwritten
by AC_CHECK_LIB if there are openssl headers available in the PATH.

Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1452436639-16838-1-git-send-email-steffan@karger.me>
URL: http://article.gmane.org/gmane.network.openvpn.devel/10978
Signed-off-by: Gert Doering <gert@greenie.muc.de>