fix/clean these Android.mk following dbort and joeo's comments
diff --git a/opengl/libs/Android.mk b/opengl/libs/Android.mk
index 33a1698..e1fd48b 100644
--- a/opengl/libs/Android.mk
+++ b/opengl/libs/Android.mk
@@ -20,8 +20,8 @@
 ifeq ($(TARGET_SIMULATOR),true)
 else
     LOCAL_SHARED_LIBRARIES += libdl
-    # we need to access the Bionic private header <bionic_tls.h>
-    LOCAL_CFLAGS += -I$(LOCAL_PATH)/../../../../bionic/libc/private
+    # we need to access the private Bionic header <bionic_tls.h>
+    LOCAL_C_INCLUDES += bionic/libc/private
 endif
 
 LOCAL_CFLAGS += -DLOG_TAG=\"libEGL\"
@@ -66,8 +66,8 @@
 ifeq ($(TARGET_SIMULATOR),true)
 else
     LOCAL_SHARED_LIBRARIES += libdl
-    # we need to access the Bionic private header <bionic_tls.h>
-    LOCAL_CFLAGS += -I$(LOCAL_PATH)/../../../../bionic/libc/private
+    # we need to access the private Bionic header <bionic_tls.h>
+    LOCAL_C_INCLUDES += bionic/libc/private
 endif
 
 LOCAL_CFLAGS += -DLOG_TAG=\"libGLESv1\"
@@ -95,8 +95,8 @@
 ifeq ($(TARGET_SIMULATOR),true)
 else
     LOCAL_SHARED_LIBRARIES += libdl
-    # we need to access the Bionic private header <bionic_tls.h>
-    LOCAL_CFLAGS += -I$(LOCAL_PATH)/../../../../bionic/libc/private
+    # we need to access the private Bionic header <bionic_tls.h>
+    LOCAL_C_INCLUDES += bionic/libc/private
 endif
 
 LOCAL_CFLAGS += -DLOG_TAG=\"libGLESv2\"