Add the math lib because the use of "exp" function

Author: mingodad

tests/Makefile

@@ -1,13 +1,13 @@
 all: nn-test-1 nn-test-2 nn-benchmark
 
 nn-test-1: nn-test-1.c ../nn.c ../nn.h
-	$(CC) nn-test-1.c ../nn.c -Wall -W -O2 -o nn-test-1
+	$(CC) nn-test-1.c ../nn.c -Wall -W -O2 -o nn-test-1 -lm
 
 nn-test-2: nn-test-2.c ../nn.c ../nn.h
-	$(CC) nn-test-2.c ../nn.c -Wall -W -O2 -o nn-test-2
+	$(CC) nn-test-2.c ../nn.c -Wall -W -O2 -o nn-test-2 -lm
 
 nn-benchmark: nn-benchmark.c ../nn.c ../nn.h
-	$(CC) -DUSE_SSE nn-benchmark.c ../nn.c -Wall -W -O3 -o nn-benchmark
+	$(CC) -DUSE_SSE nn-benchmark.c ../nn.c -Wall -W -O3 -o nn-benchmark -lm
 
 clean:
 	rm -f nn-test-1 nn-test-2 nn-benchmark