Add crypto backend for signal-protocol

configure.ac

@@ -275,6 +275,14 @@ if test "x$enable_omemo" != xno; then
         [AS_IF([test "x$enable_omemo" = xyes],
             [AC_MSG_ERROR([libsignal-protocol-c is required for omemo support])],
             [AC_MSG_NOTICE([libsignal-protocol-c not found, omemo support not enabled])])])
+
+    AC_CHECK_LIB([sodium], [sodium_init],
+        [AM_CONDITIONAL([BUILD_OMEMO], [true])
+         AC_DEFINE([HAVE_LIBSIGNAL_PROTOCOL], [1], [Have omemo]),
+         LIBS="-lsodium $LIBS"],
+        [AS_IF([test "x$enable_omemo" = xyes],
+            [AC_MSG_ERROR([libsodium is required for omemo support])],
+            [AC_MSG_NOTICE([libsodium not found, omemo support not enabled])])])
 fi
 
 AS_IF([test "x$with_themes" = xno],