Fix build errors under Ubuntu (gcc being pedantic)

src/Makefile

@@ -9,11 +9,11 @@ all: fsk_demod drs232_ldpc
 	@echo "Binaries copied to ../rx/"
 
 fsk_demod: fsk_demod.o fsk.o kiss_fft.o
-	@gcc  -lm -o fsk_demod fsk_demod.o fsk.o kiss_fft.o
+	@gcc  -o fsk_demod fsk_demod.o fsk.o kiss_fft.o -lm
 	@echo "Built fsk_demod."
 
 drs232_ldpc: drs232_ldpc.o mpdecode_core.o phi0.o
-	@gcc -lm -o drs232_ldpc drs232_ldpc.o mpdecode_core.o phi0.o
+	@gcc -o drs232_ldpc drs232_ldpc.o mpdecode_core.o phi0.o -lm
 	@echo "Built drs232_ldpc"
 
 .c.o:	$(CC)  $(CFLAGS) -c $< -o $@