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/zlib.tmpl

@@ -1,7 +1,7 @@
 # Template build file for 'zlib'
 pkgname=zlib
 version=1.2.3
-distfiles="http://www.zlib.net/$pkgname-$version@.tar.bz2"
+distfiles="http://www.zlib.net/$pkgname-$version.tar.bz2"
 build_style=configure
 CFLAGS="-fPIC -DPIC"
 configure_args="-s"