Don't try to build the emulator on Mac OS if the right tools aren't installed.

Change-Id: I7f8dcec1715657d44c562427d1c3dbd290f14ff8
diff --git a/Makefile.android b/Makefile.android
index 817c562..905622e 100644
--- a/Makefile.android
+++ b/Makefile.android
@@ -1,3 +1,5 @@
+ifeq ($(BUILD_EMULATOR),true)
+
 ifneq (,$(filter $(TARGET_ARCH),arm x86 mips))
 LOCAL_PATH:= $(call my-dir)
 
@@ -328,3 +330,4 @@
 ## VOILA!!
 
 endif  # TARGET_ARCH == arm || TARGET_ARCH == x86 || TARGET_ARCH == mips
+endif  # BUILD_EMULATOR