Add OMEMO message encryption and decryption

configure.ac

@@ -283,6 +283,14 @@ if test "x$enable_omemo" != xno; then
         [AS_IF([test "x$enable_omemo" = xyes],
             [AC_MSG_ERROR([libsodium is required for omemo support])],
             [AC_MSG_NOTICE([libsodium not found, omemo support not enabled])])])
+
+    AC_CHECK_LIB([gcrypt], [gcry_check_version],
+        [AM_CONDITIONAL([BUILD_OMEMO], [true])
+         AC_DEFINE([HAVE_OMEMO], [1], [Have omemo]),
+         LIBS="-lgcrypt $LIBS"],
+        [AS_IF([test "x$enable_omemo" = xyes],
+            [AC_MSG_ERROR([gcrypt is required for omemo support])],
+            [AC_MSG_NOTICE([gcrypt not found, omemo support not enabled])])])
 fi
 
 AS_IF([test "x$with_themes" = xno],