build: fix bitcoin-config.h regeneration after touching build files

This was a long-standing and annoying problem.

If autogen.sh was not manually run after touching configure.ac,
bitcoin-config.h would not be properly regenerated. This causes very subtle
problems when configure appears to enable a new value, but it does not end up
reflected in the build.

src/Makefile.am

@@ -462,6 +462,14 @@ DISTCLEANFILES = obj/build.h
 
 EXTRA_DIST = $(CTAES_DIST)
 
+
+config/bitcoin-config.h: config/stamp-h1
+	@$(MAKE) -C $(top_builddir) $(subdir)/$(@)
+config/stamp-h1: $(top_srcdir)/$(subdir)/config/bitcoin-config.h.in $(top_builddir)/config.status
+	$(AM_V_at)$(MAKE) -C $(top_builddir) $(subdir)/$(@)
+$(top_srcdir)/$(subdir)/config/bitcoin-config.h.in:  $(am__configure_deps)
+	$(AM_V_at)$(MAKE) -C $(top_srcdir) $(subdir)/config/bitcoin-config.h.in
+
 clean-local:
 	-$(MAKE) -C secp256k1 clean
 	-$(MAKE) -C univalue clean