As libssl depends on libcrypto, -lssl needs to precede -lcrypto in
linker invocations or else the build will fail with static OpenSSL
libraries. Adjust m4/ax_check_openssl.m4 to prevent this issue from
getting triggered when pkg-config files for OpenSSL are not available.
(cherry picked from commit 9223c7d599)
The ax_check_openssl m4 macro used OPENSSL_INCLUDES. Rename the
subst variable to OPENSSL_CFLAGS and wrap AX_CHECK_OPENSSL() in
action-if-not-found part of PKG_CHECK_MODULE check for libcrypto.
Pull and use several autoconf archive convenience macros to simplify
configure.ac.
* AX_CHECK_COMPILE_FLAG(FLAG, ...) - check whether given CFLAG works
* AX_CHECK_LINK_FLAG(FLAG, ...) - check whether given LDFLAG works
* AX_CHECK_PREPROC_FLAG(FLAG, ...) - check whether give CPPFLAG works
* AX_SAVE_FLAGS/AX_RESTORE_FLAGS - save and restore *FLAGS
