Auto merge of #2652 - str4d:820-libsnark-subtree, r=str4d

Move libsnark in-repo as a git subtree

This PR pulls in the libsnark subtree at the exact commit that we currently fetch via the depends system. To verify:

```
$ ./contrib/devtools/git-subtree-check.sh src/snark
src/snark in HEAD was last updated to upstream commit 9ada3f84ab484c57b2247c2f41091fd6a0916573 (tree c10a38c759)
src/snark in HEAD currently refers to tree 34e916d3f6
:100644 100644 427f4f4ce9 13e54da68b M	Makefile
:040000 040000 42f29e42d1 dd73536163 M	src
FAIL: subtree directory tree doesn't match subtree commit tree
```

This shows that there are changes relative to what we currently use, due to the later commits in the PR. If we exclude them, we see that the code is identical:

```
$ git checkout 26a8f68ea8
$ ./contrib/devtools/git-subtree-check.sh src/snark
src/snark in HEAD was last updated to upstream commit 9ada3f84ab484c57b2247c2f41091fd6a0916573 (tree c10a38c759)
src/snark in HEAD currently refers to tree c10a38c759
GOOD
```

Closes #820.

src/Makefile.am

@@ -1,4 +1,4 @@
-DIST_SUBDIRS = secp256k1 univalue
+DIST_SUBDIRS = secp256k1 snark univalue
 AM_LDFLAGS = $(PTHREAD_CFLAGS) $(LIBTOOL_LDFLAGS)
 
 
@@ -21,6 +21,8 @@ BITCOIN_CONFIG_INCLUDES=-I$(builddir)/config
 BITCOIN_INCLUDES=-I$(builddir) -I$(builddir)/obj $(BOOST_CPPFLAGS) $(LEVELDB_CPPFLAGS) $(CRYPTO_CFLAGS) $(SSL_CFLAGS)
 
 BITCOIN_INCLUDES += -I$(srcdir)/secp256k1/include
+BITCOIN_INCLUDES += -I$(srcdir)/snark/build/include
+BITCOIN_INCLUDES += -I$(srcdir)/snark/build/include/libsnark
 BITCOIN_INCLUDES += -I$(srcdir)/univalue/include
 
 LIBBITCOIN_SERVER=libbitcoin_server.a
@@ -30,12 +32,28 @@ LIBBITCOIN_CLI=libbitcoin_cli.a
 LIBBITCOIN_UTIL=libbitcoin_util.a
 LIBBITCOIN_CRYPTO=crypto/libbitcoin_crypto.a
 LIBSECP256K1=secp256k1/libsecp256k1.la
+LIBSNARK=snark/build/lib/libsnark.a
 LIBUNIVALUE=univalue/libunivalue.la
 LIBZCASH=libzcash.a
 
 $(LIBSECP256K1): $(wildcard secp256k1/src/*) $(wildcard secp256k1/include/*)
 	$(AM_V_at)$(MAKE) $(AM_MAKEFLAGS) -C $(@D) $(@F)
 
+# A phony target is included here to ensure libsnark is built first, so that its
+# header files are collated for use in later build steps.
+# See https://stackoverflow.com/a/10726725
+-include collate-libsnark
+collate-libsnark: $(LIBSNARK)
+
+LIBSNARK_CXXFLAGS = -fPIC -DBINARY_OUTPUT -DNO_PT_COMPRESSION=1
+LIBSNARK_CONFIG_FLAGS = CURVE=ALT_BN128 MULTICORE=1 NO_PROCPS=1 NO_DOCS=1 STATIC=1 NO_SUPERCOP=1 FEATUREFLAGS=-DMONTGOMERY_OUTPUT NO_COPY_DEPINST=1
+
+$(LIBSNARK): $(wildcard snark/src/*)
+	$(AM_V_at) CXXFLAGS="$(LIBSNARK_CXXFLAGS)" $(MAKE) $(AM_MAKEFLAGS) -C snark/ install PREFIX=$(srcdir)/build DEPINST=$(prefix) $(LIBSNARK_CONFIG_FLAGS) OPTFLAGS="-O2 -march=x86-64"
+
+libsnark-tests: $(wildcard snark/src/*)
+	$(AM_V_at) CXXFLAGS="$(LIBSNARK_CXXFLAGS)" $(MAKE) $(AM_MAKEFLAGS) -C snark/ check PREFIX=$(srcdir)/build DEPINST=$(prefix) $(LIBSNARK_CONFIG_FLAGS) OPTFLAGS="-O2 -march=x86-64"
+
 $(LIBUNIVALUE): $(wildcard univalue/lib/*)
 	$(AM_V_at)$(MAKE) $(AM_MAKEFLAGS) -C univalue/
 
@@ -89,7 +107,7 @@ LIBZCASH_H = \
   zcash/util.h \
   zcash/Zcash.h
 
-.PHONY: FORCE check-symbols check-security
+.PHONY: FORCE collate-libsnark check-symbols check-security
 # bitcoin core #
 BITCOIN_CORE_H = \
   addrman.h \
@@ -396,6 +414,7 @@ zcashd_LDADD = \
   $(LIBBITCOIN_UTIL) \
   $(LIBBITCOIN_CRYPTO) \
   $(LIBZCASH) \
+  $(LIBSNARK) \
   $(LIBLEVELDB) \
   $(LIBMEMENV) \
   $(LIBSECP256K1)
@@ -415,7 +434,6 @@ zcashd_LDADD += \
   $(CRYPTO_LIBS) \
   $(EVENT_PTHREADS_LIBS) \
   $(EVENT_LIBS) \
-  $(LIBZCASH) \
   $(LIBBITCOIN_CRYPTO) \
   $(LIBZCASH_LIBS)
 
@@ -441,6 +459,7 @@ zcash_cli_LDADD = \
   $(CRYPTO_LIBS) \
   $(EVENT_LIBS) \
   $(LIBZCASH) \
+  $(LIBSNARK) \
   $(LIBBITCOIN_CRYPTO) \
   $(LIBZCASH_LIBS)
 #
@@ -461,6 +480,7 @@ zcash_tx_LDADD = \
   $(LIBBITCOIN_UTIL) \
   $(LIBSECP256K1) \
   $(LIBZCASH) \
+  $(LIBSNARK) \
   $(LIBBITCOIN_CRYPTO) \
   $(LIBZCASH_LIBS)
 
@@ -531,6 +551,7 @@ EXTRA_DIST = leveldb
 clean-local:
 	-$(MAKE) -C leveldb clean
 	-$(MAKE) -C secp256k1 clean
+	-$(MAKE) -C snark clean
 	rm -f leveldb/*/*.gcno leveldb/helpers/memenv/*.gcno
 	-rm -f config.h