configure: fix static link with openssl

Use pkg-config to find openssl libraries, that will return all the
dependent libraries of openssl as well (e.g. -lz) that can not be
deduced in case of a static link.

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

---
Patch applied upstream:
    https://github.com/mikebrady/shairport-sync/commit/2a430ec434674ed8c1959698c061662bb7abc1a1

diff -durN shairport-sync-2.1.8.orig/configure.ac shairport-sync-2.1.8/configure.ac
--- shairport-sync-2.1.8.orig/configure.ac	2014-10-25 18:03:13.000000000 +0200
+++ shairport-sync-2.1.8/configure.ac	2014-11-29 12:52:41.507981720 +0100
@@ -21,6 +21,8 @@
 AC_PROG_CC
 AC_PROG_INSTALL
 
+PKG_PROG_PKG_CONFIG([0.9.0])
+
 # Checks for libraries.
 if test "x${with_os_type}" = xlinux; then
  AC_CHECK_LIB([rt],[clock_gettime], , AC_MSG_ERROR(librt needed))
@@ -52,8 +56,14 @@
     AC_MSG_ERROR(choose either "openssl" or "polarssl" encryption)
   fi
   if test "x${with_ssl}" = xopenssl ; then
-    AC_CHECK_LIB([crypto], [MD5_Init], , AC_MSG_ERROR(libcrypto selected, but the library cannot be found!))
-    AC_CHECK_LIB([ssl],[SSL_library_init], , AC_MSG_ERROR(openssl selected but the library cannot be found!))
+    PKG_CHECK_MODULES(
+        [CRYPTO], [libcrypto],
+        [LIBS="${CRYPTO_LIBS} ${LIBS}"
+         AC_DEFINE([HAVE_LIBCRYPTO],[1],[Define to 1 if you have libcrypto])])
+    PKG_CHECK_MODULES(
+        [SSL], [libssl],
+        [LIBS="${SSL_LIBS} ${LIBS}"
+         AC_DEFINE([HAVE_LIBSSL],[1],[Define to 1 if you have libssl])])
   elif test "x${with_ssl}" = xpolarssl ; then
     AC_CHECK_LIB([polarssl],[ssl_init], , AC_MSG_ERROR(PolarSSL selected but the library cannot be found!))
   else