build: fix MacOSX build after ec41342

Fix regression introduced in ec41342.

Also use a less ugly solution, by defining the value of
`MOC_DEFS` in the configure script instead of `Makefile.include`.

src/Makefile.include

@@ -12,7 +12,6 @@ LIBBITCOIN_CLI=$(top_builddir)/src/libbitcoin_cli.a
 LIBLEVELDB=$(top_builddir)/src/leveldb/libleveldb.a
 LIBMEMENV=$(top_builddir)/src/leveldb/libmemenv.a
 LIBBITCOINQT=$(top_builddir)/src/qt/libbitcoinqt.a
-MOC_DEFS=$(DEFS) -I$(top_srcdir)/src
 
 $(LIBBITCOIN):
 	$(MAKE) -C $(top_builddir)/src $(@F)