build: Fix version of gettext macros

Commit c9cd419c added copying of the
makefile for translation files from gnulib. The makefile from gnulib is
of version 0.18 but the build configuration cretes macros from version
0.17 which breaks the build with message:

*** error: gettext infrastructure mismatch: using a Makefile.in.in from
gettext version 0.18 but the autoconf macros are from gettext version
0.17

bootstrap.conf

@@ -195,7 +195,7 @@ buildreq="\
 autoconf   2.59
 automake   1.9.6
 autopoint  -
-gettext    0.17
+gettext    0.18
 git        1.5.5
 gzip       -
 libtool    -

configure.ac

@@ -2300,7 +2300,7 @@ dnl compute the difference between save_CPPFLAGS and CPPFLAGS and append it
 dnl to INCLUDES in order to preserve changes made by gettext but in a place
 dnl that does not break the build
 save_CPPFLAGS="$CPPFLAGS"
-AM_GNU_GETTEXT_VERSION([0.17])
+AM_GNU_GETTEXT_VERSION([0.18])
 AM_GNU_GETTEXT([external])
 GETTEXT_CPPFLAGS=
 if test "x$save_CPPFLAGS" != "x$CPPFLAGS"; then