Add standalone implementation

Add standalone implementation, giving a new choice to avoid using deprecated libgnome-keyring

src/Makefile

 
 CXX ?= g++
-# Accepts CXXSTD >= C++14
-CXXFLAGS := $(shell pkg-config --cflags --libs gnome-keyring-1) -I ./lib -I . -std=c++14
-EXTRA_FLAGS ?=
+CXXFLAGS := -I ./lib -I . -std=c++17 -DKEYRING_IMPL=$(KEYRING_IMPL)
+
+ifeq ($(KEYRING_IMPL),lib)
+    CXXFLAGS += $(shell pkg-config --cflags --libs gnome-keyring-1)
+else ifneq ($(KEYRING_IMPL),standalone)
+$(error "KEYRING_IMPL must be set to 'lib' or 'standalone'. Example: 'make KEYRING_IMPL=standalone'")
+endif
 
 secret:
 	mkdir -p ../bin/
-	$(CXX)  unlock_keyrings.cc -o ../bin/unlock_keyrings $(CXXFLAGS) $(EXTRA_FLAGS)
+	$(CXX) unlock_keyrings.cc -o ../bin/unlock_keyrings $(CXXFLAGS)