diff --git a/Makefile b/Makefile
index 632b157..57cdd7e 100644
--- a/Makefile
+++ b/Makefile
@@ -37,7 +37,7 @@ CONFIG_LTO=y
 #CONFIG_COSMO=y
 
 # installation directory
-prefix=/usr/local
+PREFIX?=/usr/local
 
 # use the gprof profiler
 #CONFIG_PROFILE=y
@@ -56,20 +56,20 @@ endif
 
 ifdef CONFIG_WIN32
   ifdef CONFIG_M32
-    CROSS_PREFIX=i686-w64-mingw32-
+    CROSS_PREFIX?=i686-w64-mingw32-
   else
-    CROSS_PREFIX=x86_64-w64-mingw32-
+    CROSS_PREFIX?=x86_64-w64-mingw32-
   endif
   EXE=.exe
 else
-  CROSS_PREFIX=
+  CROSS_PREFIX?=
   EXE=
 endif
 
 ifdef CONFIG_CLANG
   HOST_CC=clang
   CC=$(CROSS_PREFIX)clang
-  CFLAGS=-g -Wall -MMD -MF $(OBJDIR)/$(@F).d
+  CFLAGS+=-g -Wall -MMD -MF $(OBJDIR)/$(@F).d
   CFLAGS += -Wextra
   CFLAGS += -Wno-sign-compare
   CFLAGS += -Wno-missing-field-initializers
@@ -98,7 +98,7 @@ else ifdef CONFIG_COSMO
 else
   HOST_CC=gcc
   CC=$(CROSS_PREFIX)gcc
-  CFLAGS=-g -Wall -MMD -MF $(OBJDIR)/$(@F).d
+  CFLAGS+=-g -Wall -MMD -MF $(OBJDIR)/$(@F).d
   CFLAGS += -Wno-array-bounds -Wno-format-truncation
   ifdef CONFIG_LTO
     AR=$(CROSS_PREFIX)gcc-ar
@@ -125,9 +125,9 @@ CFLAGS_SMALL=$(CFLAGS) -Os
 CFLAGS_OPT=$(CFLAGS) -O2
 CFLAGS_NOLTO:=$(CFLAGS_OPT)
 ifdef CONFIG_COSMO
-LDFLAGS=-s # better to strip by default
+LDFLAGS+=-s # better to strip by default
 else
-LDFLAGS=-g
+LDFLAGS+=-g
 endif
 ifdef CONFIG_LTO
 CFLAGS_SMALL+=-flto
@@ -221,11 +221,11 @@ $(QJSC): $(OBJDIR)/qjsc.host.o \
 
 endif #CROSS_PREFIX
 
-QJSC_DEFINES:=-DCONFIG_CC=\"$(QJSC_CC)\" -DCONFIG_PREFIX=\"$(prefix)\"
+QJSC_DEFINES:=-DCONFIG_CC=\"$(QJSC_CC)\" -DCONFIG_PREFIX=\"$(PREFIX)\"
 ifdef CONFIG_LTO
 QJSC_DEFINES+=-DCONFIG_LTO
 endif
-QJSC_HOST_DEFINES:=-DCONFIG_CC=\"$(HOST_CC)\" -DCONFIG_PREFIX=\"$(prefix)\"
+QJSC_HOST_DEFINES:=-DCONFIG_CC=\"$(HOST_CC)\" -DCONFIG_PREFIX=\"$(PREFIX)\"
 
 $(OBJDIR)/qjsc.o: CFLAGS+=$(QJSC_DEFINES)
 $(OBJDIR)/qjsc.host.o: CFLAGS+=$(QJSC_HOST_DEFINES)
@@ -318,17 +318,17 @@ clean:
 	rm -rf run-test262-debug run-test262-32
 
 install: all
-	mkdir -p "$(DESTDIR)$(prefix)/bin"
+	mkdir -p "$(DESTDIR)$(PREFIX)/bin"
 	$(STRIP) qjs qjsc
-	install -m755 qjs qjsc "$(DESTDIR)$(prefix)/bin"
-	ln -sf qjs "$(DESTDIR)$(prefix)/bin/qjscalc"
-	mkdir -p "$(DESTDIR)$(prefix)/lib/quickjs"
-	install -m644 libquickjs.a "$(DESTDIR)$(prefix)/lib/quickjs"
+	install -m755 qjs qjsc "$(DESTDIR)$(PREFIX)/bin"
+	ln -sf qjs "$(DESTDIR)$(PREFIX)/bin/qjscalc"
+	mkdir -p "$(DESTDIR)$(PREFIX)/lib/quickjs"
+	install -m644 libquickjs.a "$(DESTDIR)$(PREFIX)/lib/quickjs"
 ifdef CONFIG_LTO
-	install -m644 libquickjs.lto.a "$(DESTDIR)$(prefix)/lib/quickjs"
+	install -m644 libquickjs.lto.a "$(DESTDIR)$(PREFIX)/lib/quickjs"
 endif
-	mkdir -p "$(DESTDIR)$(prefix)/include/quickjs"
-	install -m644 quickjs.h quickjs-libc.h "$(DESTDIR)$(prefix)/include/quickjs"
+	mkdir -p "$(DESTDIR)$(PREFIX)/include/quickjs"
+	install -m644 quickjs.h quickjs-libc.h "$(DESTDIR)$(PREFIX)/include/quickjs"
 
 ###############################################################################
 # examples