silent glibc compilation and redirect the log into a file

LibOS/Makefile

@@ -14,7 +14,9 @@ all: $(GLIBC_TARGET)
 
 ifeq ($(SYS),x86_64-linux-gnu)
 $(GLIBC_TARGET): $(BUILD_DIR)/Makefile
-	cd $(BUILD_DIR) && $(MAKE)
+	@echo "Building glibc, may take a while to finish. Warning messages may show up. If this process terminates with failures, see \"$(BUILD_DIR)/build.log\" for more information."
+	@rm -f $(BUILD_DIR)/build.log
+	cd $(BUILD_DIR) && (($(MAKE) >> build.log) 2>&1 | tee -a build.log)
 
 $(BUILD_DIR)/Makefile: $(addprefix $(GLIBC_SRC)/,configure elf/Versions nptl/Versions dlfcn/Versions)
 ifeq ($(DEBUG),1)