build: add option for reducing exports

Enabled automatically if boost >= 1.49.
See: https://svn.boost.org/trac/boost/ticket/2309

Also, check for a default visibility attribute, so that we can mark future
api functions correctly.

src/Makefile.am

@@ -262,6 +262,7 @@ endif
 
 bitcoind_LDADD += $(BOOST_LIBS) $(BDB_LIBS) $(SSL_LIBS) $(CRYPTO_LIBS)
 bitcoind_CPPFLAGS = $(BITCOIN_INCLUDES)
+bitcoind_LDFLAGS = $(RELDFLAGS) $(AM_LDFLAGS)
 
 # bitcoin-cli binary #
 bitcoin_cli_LDADD = \
@@ -299,10 +300,12 @@ endif
 bitcoin_tx_SOURCES = bitcoin-tx.cpp
 bitcoin_tx_CPPFLAGS = $(BITCOIN_INCLUDES)
 #
+bitcoin_tx_LDFLAGS = $(RELDFLAGS) $(AM_LDFLAGS)
 
 if TARGET_WINDOWS
 bitcoin_cli_SOURCES += bitcoin-cli-res.rc
 endif
+bitcoin_cli_LDFLAGS = $(RELDFLAGS) $(AM_LDFLAGS)
 
 CLEANFILES = leveldb/libleveldb.a leveldb/libmemenv.a *.gcda *.gcno