--- a/doc/po4a.mk
+++ b/doc/po4a.mk
@@ -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.