diff --git a/gnu/packages/synergy.scm b/gnu/packages/synergy.scm
index 367bb323b3..739ea3bd75 100644
--- a/gnu/packages/synergy.scm
+++ b/gnu/packages/synergy.scm
@@ -63,7 +63,8 @@ (define-public synergy
            (lambda* (#:key inputs #:allow-other-keys)
              (setenv "CPLUS_INCLUDE_PATH"
                      (string-append (assoc-ref inputs "avahi")
-                                    "/include/avahi-compat-libdns_sd"))
+                                    "/include/avahi-compat-libdns_sd:"
+                                    (or (getenv "CPLUS_INCLUDE_PATH") "")))
              #t)))))
     (inputs
      `(("avahi" ,avahi)