gnu: unzip: Fix CVE-2014-9636 and some other bugs.

* gnu/packages/patches/unzip-CVE-2014-9636.patch,
  gnu/packages/patches/unzip-allow-greater-hostver-values.patch,
  gnu/packages/patches/unzip-increase-size-of-cfactorstr.patch,
  gnu/packages/patches/unzip-initialize-symlink-flag.patch,
  gnu/packages/patches/unzip-remove-build-date.patch: New files.
* gnu-system.am (dist_patch_DATA): Add them.
* gnu/packages/zip.scm (unzip)[source]: Add patches.
  [arguments]: Use 'modify-phases'.  Remove custom 'configure' phase; pass
  additional make-flags instead.  Add custom 'build' phase that builds
  "generic_gcc" target; remove "generic_gcc" from make-flags.

gnu/packages/patches/unzip-remove-build-date.patch

@@ -0,0 +1,19 @@
+Copied from Debian.
+
+From: Jérémy Bobbio <lunar@debian.org>
+Subject: Remove build date
+Bug-Debian: http://bugs.debian.org/782851
+ In order to make unzip build reproducibly, we remove the
+ (already optional) build date from the binary.
+
+--- a/unix/unix.c
++++ b/unix/unix.c
+@@ -1705,7 +1705,7 @@
+ #endif /* Sun */
+ #endif /* SGI */
+ 
+-#ifdef __DATE__
++#if 0
+       " on ", __DATE__
+ #else
+       "", ""