Add support for generic Makefile variables

Makefile

@@ -9,6 +9,7 @@ SRC_DIR ?= src
 SDK_DIR ?= sdk
 OUT_DIR ?= build
 LIB_DIR ?= lib
+DESTDIR ?=
 
 # Project definition
 include project.mk
@@ -78,8 +79,12 @@ default: library
 .PHONY:  all dirs clean doc install
 
 install: library
-	cp $(OUT_DIR)/lib$(PROJECT_NAME).so $(PREFIX)/$(LIB_DIR)/
-	cp -R include/aisl $(PREFIX)/include/
+	$(info installing files)
+	@mkdir -p $(DESTDIR)$(PREFIX)/$(LIB_DIR)
+	@mkdir -p $(DESTDIR)$(PREFIX)/include
+
+	@cp $(OUT_DIR)/lib$(PROJECT_NAME).so $(DESTDIR)$(PREFIX)/$(LIB_DIR)
+	@cp -R include/aisl $(DESTDIR)$(PREFIX)/include
 
 # vim:ft=make
 #