Fix etc1tool on Mac SDK: -lz was missing.

Change-Id: I50f5500493be638182d3302cae2e2385722da22d
diff --git a/tools/etc1tool/Android.mk b/tools/etc1tool/Android.mk
index 37aab14..a66af25 100644
--- a/tools/etc1tool/Android.mk
+++ b/tools/etc1tool/Android.mk
@@ -24,16 +24,15 @@
 	libETC1
 
 ifeq ($(HOST_OS),linux)
-LOCAL_LDLIBS := -lz -lrt
+LOCAL_LDLIBS += -lrt
 endif
 
-ifeq ($(HOST_OS),windows)
-ifeq ($(strip $(USE_CYGWIN)),)
-LOCAL_LDLIBS := -lz -lws2_32
-endif
+# Statically link libz for MinGW (Win SDK under Linux),
+# and dynamically link for all others.
 ifneq ($(strip $(USE_MINGW)),)
-LOCAL_STATIC_LIBRARIES += libz
-endif
+  LOCAL_STATIC_LIBRARIES += libz
+else
+  LOCAL_LDLIBS += -lz
 endif
 
 LOCAL_MODULE := etc1tool