rtems_waf: Compiler flags appear multiple times

[Sebastian Huber]

For example:

Checking for RTEMS_DEBUG
==>
#include <rtems/score/cpuopts.h>
#ifndef RTEMS_DEBUG
 #error RTEMS_DEBUG is not defined
#endif
#if RTEMS_DEBUG
 /* RTEMS_DEBUG is true */
#else
 #error RTEMS_DEBUG is false
#endif
int main() { return 0; }

<==
[1/2] [32mc: build/.conf_check_e18ee633068d2a9f6360268e197c95ec/test.c -> build/.conf_check_e18ee633068d2a9f6360268e197c95ec/testbuild/test.c.1.o
[0m
['/opt/rtems-4.12/bin/arm-rtems4.12-gcc', '-qrtems', '-B/opt/rtems-4.12/arm-rtems4.12/lib/', '-B/opt/rtems-4.12/arm-rtems4.12/altcycv_devkit_smp/lib/', '--specs', 'bsp_specs', '-march=armv7-a', '-march=armv7-a', '-mthumb', '-mthumb', '-mfpu=neon', '-mfpu=neon', '-mfloat-abi=hard', '-mfloat-abi=hard', '-mtune=cortex-a9', '-mtune=cortex-a9', '../test.c', '-c', '-o', 'test.c.1.o']