Provide portable wrapper for qsort_r.

qsort_r() is declared differently on BSD and GNU libc and
may be absent on other platforms.

Provide qsort_r_compat() which should work everywhere.

Change-Id: I17a9026573b1ed971ff7d79895e7eb41ba108807
diff --git a/libcutils/Android.mk b/libcutils/Android.mk
index 3809733..0d49165 100644
--- a/libcutils/Android.mk
+++ b/libcutils/Android.mk
@@ -46,6 +46,7 @@
 	record_stream.c \
 	process_name.c \
 	properties.c \
+	qsort_r_compat.c \
 	threads.c \
 	sched_policy.c \
 	iosched_policy.c \