Makefile: honor config when building gen_context

The passes $(DEFS) (which should literally be "-DHAVE_CONFIG_H") to the
compiler and $(COMMON_LIB) (which is necessary when using external ASM
to the linker) when building gen_context.

Makefile.am

@@ -126,13 +126,13 @@ endif
 if USE_ECMULT_STATIC_PRECOMPUTATION
 CPPFLAGS_FOR_BUILD +=-I$(top_srcdir) -I$(builddir)/src
 
-gen_context_OBJECTS = gen_context.o
+gen_context_OBJECTS = $(COMMON_LIB) gen_context.o
 gen_context_BIN = gen_context$(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 $@
+	$(LIBTOOL) --tag=CC $(LIBTOOLFLAGS) --mode=link $(CC_FOR_BUILD) $(CFLAGS_FOR_BUILD) $(LDFLAGS_FOR_BUILD) $^ -o $@
 
 $(libsecp256k1_la_OBJECTS): src/ecmult_static_context.h
 $(tests_OBJECTS): src/ecmult_static_context.h