Separate test function and main()

This allows for inclusion of the test function/file in a program with an
existing main().  This is an isolated piece of #19.

makefile

@@ -1,6 +1,6 @@
 CC=gcc
 CFLAGS=-Wall --std=c99 
-SRC=siphash.c halfsiphash.c test.c
+SRC=siphash.c halfsiphash.c test.c testmain.c
 BIN=test debug vectors
 
 ifneq ($(cROUNDS),)

test.c

@@ -43,7 +43,7 @@ const char *labels[4] = {
 
 size_t lengths[4] = {8, 16, 4, 8};
 
-int main() {
+int siphash_test() {
     uint8_t in[64], out[16], k[16];
     int i;
 #ifndef GETVECTORS

testmain.c

@@ -0,0 +1,5 @@
+int siphash_test();
+
+int main(void) {
+    return siphash_test();
+}