go: build libgo dependencies automatically via LIBS variable (backtrace)

Issue genodelabs#304

lib/mk/backtrace.mk

@@ -1,14 +1,12 @@
-MY_BUILD_DIR := $(LIB_CACHE_DIR)/backtrace
-MY_TARGET := $(MY_BUILD_DIR)/.libs/libbacktrace.a
+LIBS = libbacktrace
 
-# to glue gnu_build.mk
-CUSTOM_TARGET_DEPS := finished.tag
+LIBBACKTRACE_A := $(LIB_CACHE_DIR)/libbacktrace/.libs/libbacktrace.a
 
-$(MY_TARGET): built.tag
-
-finished.tag: $(MY_TARGET)
-	@$(MSG_INST)$*
-	ln -sf $(MY_TARGET) $(MY_BUILD_DIR)/backtrace.lib.a
+finished.tag: backtrace.lib.a
+	@$(MSG_INST)$<
+	$(VERBOSE)ln -sf $(LIBBACKTRACE_A) $<
 	@touch $@
 
-include $(REP_DIR)/lib/mk/libbacktrace.inc
+ifeq ($(called_from_lib_mk),yes)
+all: finished.tag
+endif

lib/mk/libbacktrace.mk

@@ -1 +1,13 @@
-include $(REP_DIR)/lib/mk/libbacktrace.inc
+include $(REP_DIR)/lib/import/import-libbacktrace.mk
+
+LIBS += libc
+
+PKG_DIR = $(call select_from_ports,libgo)/src/lib/gcc/libbacktrace
+EXT_OBJECTS += $(shell $(CC) $(CC_MARCH) -print-file-name=libgcc_eh.a)
+
+# to glue gnu_build.mk
+CUSTOM_TARGET_DEPS += built.tag
+
+CONFIGURE_ARGS += --disable-shared
+
+include $(call select_from_repositories,mk/gnu_build.mk)