--- a/doc/po4a.mk	2014-06-15 21:17:43.000000000 +0300
+++ b/doc/po4a.mk	2015-01-02 00:36:19.000000000 +0200
@@ -23,7 +23,7 @@
 # threshold is 80%), it won't be distributed, and the build won't fail.
 #
 
-mandir = $(mandir)/$(lang)
+mandir := $(mandir)/$(lang)
 
 # Inform automake that we want to install some man pages in section 1, 5
 # and 8.