[PATCH]: fix man pages error in po4a.mk

[Andreas Oberritter]

| for po in `ls -1 ./*.pt_BR.po 2>/dev/null`; do \
|               make $(basename ${po%.pt_BR.po}); \
|       done
| Makefile:904: *** Recursive variable `mandir' references itself (eventually). 
 Stop.

Signed-off-by: Andreas Oberritter <address@hidden>

--- parted-3.1/doc/po4a.mk.orig 2012-03-15 14:09:11.555831872 +0100
+++ parted-3.1/doc/po4a.mk      2012-03-15 14:10:44.243830985 +0100
@@ -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.