Makefile.am: Honor config when building gen_context

This passes $(DEFS) (which should literally be "-DHAVE_CONFIG_H") to the
compiler when building gen_context.

This has currently no effect because gen_context.c does not check for
this macro but it's conceivable that it may do so in the future.

Makefile.am

@@ -129,7 +129,7 @@ CPPFLAGS_FOR_BUILD +=-I$(top_srcdir) -I$(builddir)/src
 gen_context_OBJECTS = gen_context.o
 gen_context_BIN = gen_context$(BUILD_EXEEXT)
 gen_%.o: src/gen_%.c src/libsecp256k1-config.h
-	$(CC_FOR_BUILD) $(CPPFLAGS_FOR_BUILD) $(CFLAGS_FOR_BUILD) -c $< -o $@
+	$(CC_FOR_BUILD) $(DEFS) $(CPPFLAGS_FOR_BUILD) $(CFLAGS_FOR_BUILD) -c $< -o $@
 
 $(gen_context_BIN): $(gen_context_OBJECTS)
 	$(CC_FOR_BUILD) $(CFLAGS_FOR_BUILD) $(LDFLAGS_FOR_BUILD) $^ -o $@