leveldb: integrate leveldb into our buildsystem

leveldb's buildsystem causes us a few problems:
- breaks out-of-tree builds
- forces flags used for some tools
- limits cross builds

Rather than continuing to add wrappers around it, simply integrate it into our
build.

src/Makefile.am

@@ -3,6 +3,7 @@ DIST_SUBDIRS = secp256k1 univalue
 AM_LDFLAGS = $(PTHREAD_CFLAGS) $(LIBTOOL_LDFLAGS) $(HARDENED_LDFLAGS)
 AM_CXXFLAGS = $(HARDENED_CXXFLAGS)
 AM_CPPFLAGS = $(HARDENED_CPPFLAGS)
+EXTRA_LIBRARIES =
 
 if EMBEDDED_UNIVALUE
 LIBUNIVALUE = univalue/libunivalue.la
@@ -13,21 +14,6 @@ else
 LIBUNIVALUE = $(UNIVALUE_LIBS)
 endif
 
-if EMBEDDED_LEVELDB
-LEVELDB_CPPFLAGS += -I$(srcdir)/leveldb/include
-LEVELDB_CPPFLAGS += -I$(srcdir)/leveldb/helpers/memenv
-LIBLEVELDB += $(builddir)/leveldb/libleveldb.a
-LIBMEMENV  += $(builddir)/leveldb/libmemenv.a
-
-# NOTE: This dependency is not strictly necessary, but without it make may try to build both in parallel, which breaks the LevelDB build system in a race
-$(LIBLEVELDB): $(LIBMEMENV)
-
-$(LIBLEVELDB) $(LIBMEMENV):
-	@echo "Building LevelDB ..." && $(MAKE) -C $(@D) $(@F) CXX="$(CXX)" \
-	  CC="$(CC)" PLATFORM=$(TARGET_OS) AR="$(AR)" $(LEVELDB_TARGET_FLAGS) \
-          OPT="$(AM_CXXFLAGS) $(PIE_FLAGS) $(CXXFLAGS) $(AM_CPPFLAGS) $(CPPFLAGS) -D__STDC_LIMIT_MACROS"
-endif
-
 BITCOIN_CONFIG_INCLUDES=-I$(builddir)/config
 BITCOIN_INCLUDES=-I$(builddir) -I$(builddir)/obj $(BOOST_CPPFLAGS) $(LEVELDB_CPPFLAGS) $(CRYPTO_CFLAGS) $(SSL_CFLAGS)
 
@@ -49,7 +35,7 @@ $(LIBSECP256K1): $(wildcard secp256k1/src/*) $(wildcard secp256k1/include/*)
 
 # Make is not made aware of per-object dependencies to avoid limiting building parallelization
 # But to build the less dependent modules first, we manually select their order here:
-EXTRA_LIBRARIES = \
+EXTRA_LIBRARIES += \
   crypto/libbitcoin_crypto.a \
   libbitcoin_util.a \
   libbitcoin_common.a \
@@ -482,6 +468,10 @@ endif
 	@test -f $(PROTOC)
 	$(AM_V_GEN) $(PROTOC) --cpp_out=$(@D) --proto_path=$(<D) $<
 
+if EMBEDDED_LEVELDB
+include Makefile.leveldb.include
+endif
+
 if ENABLE_TESTS
 include Makefile.test.include
 endif