Honor PREFIX environment variable

makefile

@@ -6,5 +6,5 @@ clean distclean:
 
 build/Makefile: CMakeLists.txt
 	mkdir -p build
-	cd build && cmake .. -DCMAKE_BUILD_TYPE=Release $(addprefix -DINIT_FLAVOR=,$(INIT_FLAVOR)) -DCMAKE_C_FLAGS="$(CFLAGS)" $(addprefix -DCMAKE_C_COMPILER=,$(CC))
+	cd build && cmake .. -DCMAKE_BUILD_TYPE=Release $(addprefix -DINIT_FLAVOR=,$(INIT_FLAVOR)) -DCMAKE_C_FLAGS="$(CFLAGS)" $(addprefix -DCMAKE_C_COMPILER=,$(CC)) $(addprefix -DCMAKE_INSTALL_PREFIX=,$(PREFIX))