build: fix axiom's build dependency check (#1031)

Make the `HAVE_PROTOBUF_C != 0` check part of recipe where its needed
rather than making the check in a phony target. Using phony target results
in axiom rebuilding every time any target is build.

Fixup of #1012.

Author: lavarou

axiom/Makefile

@@ -195,7 +195,10 @@ nr_config.h: configure Makefile
 #
 # Build the 8T protobuf code.
 #
-v1.pb-c.c: v1.proto protobuf-c
+v1.pb-c.c: v1.proto
+ifeq (0,$(HAVE_PROTOBUF_C))
+	$(error Build dependency 'protobuf-c' not found.)
+endif
 	$(PROTOBUF_C_PREFIX)/bin/protoc-c --c_out=. $<
 
 #
@@ -205,12 +208,6 @@ v1.pb-c.c: v1.proto protobuf-c
 v1.pb-c.o: v1.pb-c.c
 	$(CC) $(AXIOM_CPPFLAGS) $(CPPFLAGS) $(AXIOM_CFLAGS) $(PCRE_CFLAGS) $(VENDOR_CFLAGS) $(CFLAGS) -MMD -MP -Wno-cast-qual -c $< -o $@
 
-.PHONY: protobuf-c
-protobuf-c:
-ifeq (0,$(HAVE_PROTOBUF_C))
-	$(error Build dependency 'protobuf-c' not found.)
-endif
-
 #
 # Track the flags passed to the compiler to force a rebuild when they change.
 # This ensures a rebuild occurs when the version number or commit are updated.