bitcoin-unlimited: fix build with boost 1.66

Use part of fix for mainline bitcoin:
1ec0c0a01c

Also enable parallel builds for speed.

pkgs/applications/altcoins/bitcoin-unlimited-const-comparators.patch

@@ -0,0 +1,38 @@
+--- a/src/txmempool.h
++++ b/src/txmempool.h
+@@ -204,7 +204,7 @@
+ class CompareTxMemPoolEntryByDescendantScore
+ {
+ public:
+-    bool operator()(const CTxMemPoolEntry& a, const CTxMemPoolEntry& b)
++    bool operator()(const CTxMemPoolEntry& a, const CTxMemPoolEntry& b) const
+     {
+         bool fUseADescendants = UseDescendantScore(a);
+         bool fUseBDescendants = UseDescendantScore(b);
+@@ -226,7 +226,7 @@
+     }
+ 
+     // Calculate which score to use for an entry (avoiding division).
+-    bool UseDescendantScore(const CTxMemPoolEntry &a)
++    bool UseDescendantScore(const CTxMemPoolEntry &a) const
+     {
+         double f1 = (double)a.GetModifiedFee() * a.GetSizeWithDescendants();
+         double f2 = (double)a.GetModFeesWithDescendants() * a.GetTxSize();
+@@ -241,7 +241,7 @@
+ class CompareTxMemPoolEntryByScore
+ {
+ public:
+-    bool operator()(const CTxMemPoolEntry& a, const CTxMemPoolEntry& b)
++    bool operator()(const CTxMemPoolEntry& a, const CTxMemPoolEntry& b) const
+     {
+         double f1 = (double)a.GetModifiedFee() * b.GetTxSize();
+         double f2 = (double)b.GetModifiedFee() * a.GetTxSize();
+@@ -255,7 +255,7 @@
+ class CompareTxMemPoolEntryByEntryTime
+ {
+ public:
+-    bool operator()(const CTxMemPoolEntry& a, const CTxMemPoolEntry& b)
++    bool operator()(const CTxMemPoolEntry& a, const CTxMemPoolEntry& b) const
+     {
+         return a.GetTime() < b.GetTime();
+     }

pkgs/applications/altcoins/bitcoin-unlimited.nix

@@ -21,8 +21,13 @@ stdenv.mkDerivation rec {
                   miniupnpc utillinux protobuf libevent ]
                   ++ optionals withGui [ qt4 qrencode ];
 
+  patches = [
+    ./bitcoin-unlimited-const-comparators.patch
+  ];
+
   configureFlags = [ "--with-boost-libdir=${boost.out}/lib" ]
                      ++ optionals withGui [ "--with-gui=qt4" ];
+  enableParallelBuilding = true;
 
   meta = {
     description = "Peer-to-peer electronic cash system (Unlimited client)";