secp256k1: Add build-side changes for libsecp256k1

Note: This is added to our existing automake targets rather than as a
libtool-style lib. The switch to libtool-style targets can come later if it
proves to not add any complications.

Makefile.am

@@ -35,6 +35,7 @@ COVERAGE_INFO = baseline_filtered_combined.info baseline.info block_test.info \
 
 dist-hook:
 	-$(MAKE) -C $(top_distdir)/src/leveldb clean
+	-$(MAKE) -C $(top_distdir)/src/secp256k1 distclean
 	-$(GIT) archive --format=tar HEAD -- src/version.cpp | $(AMTAR) -C $(top_distdir) -xf -
 
 distcheck-hook: