The previous check failed to properly find if the symbols were declared so instead
we use AC_CHECK_DECLS as would be expected.

Signed-off-by: Francisco Blas Izquierdo Riera (klondike) <klondike@gentoo.org>
--- configure.ac	2015-10-24 14:14:00.000000000 +0200
+++ configure.ac	2015-12-10 11:49:45.979880686 +0100
@@ -409,14 +409,10 @@
 				[AC_MSG_WARN([cross compiling: not checking])])
 			 ])
 
-		 AC_COMPILE_IFELSE(
-			[AC_LANG_PROGRAM([[
-#include <media.h>
-				]],
-				[[purple_media_set_encryption_parameters(NULL, "", "", "", "", 0);
-			          purple_media_set_decryption_parameters(NULL, "", "", "", "", "", 0);]]
-			 )],
-			 [AC_DEFINE(HAVE_SRTP, 1, [Define if we have SRTP support in media backend.])]
+		 AC_CHECK_DECLS([purple_media_set_encryption_parameters,purple_media_set_decryption_parameters],
+			 [AC_DEFINE(HAVE_SRTP, 1, [Define if we have SRTP support in media backend.])],
+			 [],
+			 [[#include <media.h>]]
 		 )
 
 		 CFLAGS=$ac_save_CFLAGS