Build: Use AM_V_GEN in Makefiles where appropriate

When generating new files as part of the Makefile the recipe is
sometimes suppressed with $(AM_V_GEN) and sometimes with `@`. We should
prefer $(AM_V_GEN), since this also prints the lines in silent mode.
This is arguably more in style with the current recipe echoing.

Before:
Generated test/data/script_tests.json.h
Now:
  GEN      test/data/script_tests.json.h

A side effect of this change is that the recipe for generating build.h
is now echoed on each make run. Arguably this makes its generation more
transparent.

src/Makefile.am

@@ -346,7 +346,7 @@ BITCOIN_CORE_H = \
 
 obj/build.h: FORCE
 	@$(MKDIR_P) $(builddir)/obj
-	@$(top_srcdir)/share/genbuild.sh "$(abs_top_builddir)/src/obj/build.h" \
+	$(AM_V_GEN) $(top_srcdir)/share/genbuild.sh "$(abs_top_builddir)/src/obj/build.h" \
 	  "$(abs_top_srcdir)"
 libbitcoin_util_a-clientversion.$(OBJEXT): obj/build.h
 
@@ -1085,12 +1085,11 @@ endif
 
 %.raw.h: %.raw
 	@$(MKDIR_P) $(@D)
-	@{ \
+	$(AM_V_GEN) { \
 	 echo "static unsigned const char $(*F)_raw[] = {" && \
 	 $(HEXDUMP) -v -e '8/1 "0x%02x, "' -e '"\n"' $< | $(SED) -e 's/0x  ,//g' && \
 	 echo "};"; \
 	} > "$@.new" && mv -f "$@.new" "$@"
-	@echo "Generated $@"
 
 include Makefile.minisketch.include
 

src/Makefile.test.include

@@ -414,10 +414,9 @@ endif
 
 %.json.h: %.json
 	@$(MKDIR_P) $(@D)
-	@{ \
+	$(AM_V_GEN) { \
 	 echo "namespace json_tests{" && \
 	 echo "static unsigned const char $(*F)[] = {" && \
 	 $(HEXDUMP) -v -e '8/1 "0x%02x, "' -e '"\n"' $< | $(SED) -e 's/0x  ,//g' && \
 	 echo "};};"; \
 	} > "$@.new" && mv -f "$@.new" "$@"
-	@echo "Generated $@"