configure: use pkg-config to find openssl

The current check is broken for static linking, since the openssl
libs need to be linked against libz, which is missing in the link
command when doing a static build:
    http://autobuild.buildroot.net/results/787/787cd9992b00d8008641af208f5fb7c4afb40b06/build-end.log

Instead of trying convoluted tests to decide what additional libs
are required, just delegate this job to pkg-config.

At the same time, fix a few legacy, deprecated constructs in favour
of their new counterparts.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>

---
Upstream status: submitted, but no public mailing list (sent to the
mail documented in: http://www.antisip.com/doc/exosip2/index.html )

diff -durN libeXosip2-3.6.0.orig/configure.in libeXosip2-3.6.0/configure.in
--- libeXosip2-3.6.0.orig/configure.in	2014-06-22 23:58:26.549487427 +0200
+++ libeXosip2-3.6.0/configure.in	2014-06-22 23:59:12.822008499 +0200
@@ -44,7 +44,7 @@
 AC_CONFIG_AUX_DIR(scripts)
 AC_CONFIG_HEADERS(config.h)
 AM_INIT_AUTOMAKE($PACKAGE, $VERSION)
-AM_ACLOCAL_INCLUDE(scripts)
+AC_CONFIG_MACRO_DIR([scripts])
 
 AC_CANONICAL_HOST
 
@@ -371,13 +371,15 @@
 
 AC_ARG_ENABLE(openssl,
 	[  --enable-openssl        enable support for openssl],
-	enable_openssl=$enableval,enable_openssl="yes")
+	enable_openssl=$enableval,enable_openssl="auto")
 
-if test "$enable_openssl" = "yes" ; then
-	AC_CHECK_HEADERS(openssl/ssl.h)
-	AC_CHECK_LIB(ssl,SSL_CTX_new,[EXOSIP_LIB="$EXOSIP_LIB -lssl"])
-	AC_CHECK_LIB(crypto,X509_get_issuer_name,[EXOSIP_LIB="$EXOSIP_LIB -lcrypto"])
-fi
+AS_IF([test "$enable_openssl" = "yes" -o "$enable_openssl" = "auto"],
+      [PKG_CHECK_MODULES([SSL], [openssl],
+                         [EXOSIP_LIB="$EXOSIP_LIB $SSL_LIBS"],
+                         [AS_IF([test "$enable_openssl" = "yes"],
+                                [AC_MSG_ERROR([can not find openssl])])
+                         ])
+      ])
 
 AC_ARG_ENABLE(srvrec,
 	[  --enable-srvrec         enable support for SRV records DNS queries (yes) ],
diff -durN build/libeXosip2-3.6.0.orig/Makefile.am build/libeXosip2-3.6.0/Makefile.am
--- libeXosip2-3.6.0.orig/Makefile.am	2014-06-23 00:14:25.328226745 +0200
+++ libeXosip2-3.6.0/Makefile.am	2014-06-23 00:14:38.296371236 +0200
@@ -1,4 +1,4 @@
-
+ACLOCAL_AMFLAGS = -I scripts
 EXTRA_DIST = README autogen.sh
 
 SUBDIRS = scripts platform src include tools help