configure: Look for libcrypto even when configuring --without-krb4

Signed-off-by: Anders Kaseorg <andersk@mit.edu>
configure: Only use LIBCRYPTO_{CFLAGS,LIBS} on PKG_CHECK_MODULES success

Signed-off-by: Anders Kaseorg <andersk@mit.edu>
configure: Try openssl.pc after libcrypto.pc

OpenSSL < 0.9.8 didn’t ship libcrypto.pc.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>