Fix nettle_hash() function to avoid ABI incompatibilities.

The way of accessing the list of available hashes on nettle was
vulnerable to breaking if the version of libnettle in use was
different to the version dnsmasq was compiled against.
Change to a new system if libnettle >= 3.4 is in use.
Older versions if nettle are still OK, once 3.4 is reached,
the ABi problem is fixed. Thanks to Petr Menšík for clues on this.
diff --git a/debian/changelog b/debian/changelog
index 40f6805..0ffe556 100644
--- a/debian/changelog
+++ b/debian/changelog
@@ -5,6 +5,9 @@
    * Fix exit code for dhcp_release6 (closes: #833596)
    * Add project homepage to control file. (closes: #887764)
    * New binary package dnsmasq-base-lua, includes Lua support.
+   * Remove hardwired shlibs dependency for libnettle 3.3 and
+	fix code to avoid ABI breakage as long as compiled against
+	libnettle 3.4 or later. (closes: #891315)
 
  -- Simon Kelley <simon@thekelleys.org.uk>  Fri, 16 Feb 2018 19:54:22 +0000