Makefile: make it cross-compile (and Buildroot) friendly.

The current Makefile makes heavy assumptions that it is doing native
compilation on the RPi, as it checks that `uname -m` is an ARM machine.

This is wrong in the cross-compilation case.

Remove the conditional altogether, and do not override the CFLAGS
as passed in the environment (Buildroot passes proper CFLAGS).

[intial patch by: Eric Limpens <limpens@gmail.com>]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>

diff -durN pifmrds-c67306ea9b8d827f45e0d90279d367e97119bcb1.orig/src/Makefile pifmrds-c67306ea9b8d827f45e0d90279d367e97119bcb1/src/Makefile
--- pifmrds-c67306ea9b8d827f45e0d90279d367e97119bcb1.orig/src/Makefile	2014-05-04 18:21:40.000000000 +0200
+++ pifmrds-c67306ea9b8d827f45e0d90279d367e97119bcb1/src/Makefile	2014-06-21 16:38:31.971804343 +0200
@@ -1,20 +1,8 @@
 CC = gcc
-STD_CFLAGS = -Wall -std=gnu99 -c -g -O3
 
-# Enable ARM-specific options only on ARM, and compilation of the app only on ARM
-UNAME := $(shell uname -m)
-
-ifeq ($(UNAME), armv6l)
-	CFLAGS = $(STD_CFLAGS) -march=armv6 -mtune=arm1176jzf-s -mfloat-abi=hard -mfpu=vfp -ffast-math
-
 app: rds.o waveforms.o pi_fm_rds.o fm_mpx.o control_pipe.o
 	$(CC) -o pi_fm_rds rds.o waveforms.o pi_fm_rds.o fm_mpx.o control_pipe.o -lm -lsndfile
 
-else
-	CFLAGS = $(STD_CFLAGS)
-endif
-
-
 rds_wav: rds.o waveforms.o rds_wav.o fm_mpx.o
 	$(CC) -o rds_wav rds_wav.o rds.o waveforms.o fm_mpx.o -lm -lsndfile