Change readme, and add changable build dir

Makefile

@@ -1,5 +1,7 @@
 LIB=libcx
 
+BUILD_DIR?=build
+
 COM_FLAGS = -fPIC -Isrc/include
 CC_FLAGS  = -std=c23 -ggdb
 CXX_FLAGS = -std=c++23 -nostdinc++ -ggdb -fexceptions
@@ -11,28 +13,28 @@ AR=ar
 
 cxx_sources=$(wildcard src/*.cpp)
 c_sources=$(wildcard src/*.c)
-objects=$(patsubst src/%.cpp,build/obj/%.cpp.o,$(cxx_sources)) $(patsubst src/%.c,build/obj/%.c.o,$(c_sources))
+objects=$(patsubst src/%.cpp,$(BUILD_DIR)/obj/%.cpp.o,$(cxx_sources)) $(patsubst src/%.c,$(BUILD_DIR)/obj/%.c.o,$(c_sources))
 
 
 all: shared static compile_commands.json
 
-shared: build/$(LIB).so
-static: build/$(LIB).a
+shared: $(BUILD_DIR)/$(LIB).so
+static: $(BUILD_DIR)/$(LIB).a
 
-build/$(LIB).a: $(objects)
+$(BUILD_DIR)/$(LIB).a: $(objects)
 	$(AR) rcs $@ $^  
 
-build/$(LIB).so: $(objects)
+$(BUILD_DIR)/$(LIB).so: $(objects)
 	$(LD) -shared -o $@ $^
 
 clean:
-	rm -r build/obj/
+	rm -r $(BUILD_DIR)/obj/
 
-build/obj/%.cpp.o: */%.cpp
+$(BUILD_DIR)/obj/%.cpp.o: */%.cpp
 	@mkdir -p $(dir $@)
 	$(CXX) -c -o $@ $< $(CXX_FLAGS) $(COM_FLAGS)
 
-build/obj/%.c.o: */%.c
+$(BUILD_DIR)/obj/%.c.o: */%.c
 	@mkdir -p $(dir $@)
 	$(CC) -c -o $@ $< $(CC_FLAGS) $(COM_FLAGS)