Fix compiler detection for avr-gcc with -std=gnu99

Jack

2017-02-21 fb2ee796938f75527571252128a306ba7658871a

Fix compiler detection for avr-gcc with -std=gnu99

We don't seem to have strtof, but the compiler detection says we do.
Tweaking the compiler detection code to set HAVE_STRTOF to 0 seems to
fix it.

 libscpi/inc/scpi/cc.h

@@ -116,6 +116,8 @@
#if defined(__AVR__)
#include <stdlib.h>
#define HAVE_DTOSTRE            1
#undef HAVE_STRTOF
#define HAVE_STRTOF                0
#endif

/* default values */