Use bash features to improve $distfiles parsing.

That means that it's not necessary anymore to use an '@' before the
extract suffix string. yay.

--HG--
extra : convert_revision : 83ce2c783cb917ef59b87eacb0565cfcff277218

templates/bison.tmpl

@@ -1,7 +1,7 @@
 # Template build file for 'bison'.
 pkgname=bison
 version=2.3
-distfiles="http://ftp.gnu.org/pub/gnu/bison/$pkgname-$version@.tar.bz2"
+distfiles="http://ftp.gnu.org/pub/gnu/bison/$pkgname-$version.tar.bz2"
 build_style=gnu_configure
 configure_env="ac_cv_path_M4=/usr/bin/m4"
 short_desc="GNU yacc(1) replacement"