trace-cmd: Use conditional assignment of CC and AR
trace-cmd: Use conditional assignment of CC and AR

Allow the user to specify CC and AR via the environment by only
setting them in the Makefile if they don't already exist. This is
useful for external build systems for example.

Signed-off-by: Darren Hart <38f9eaa30910c40b06f5a60020b1222f2a953801@linux.intel.com>
LKML-Reference: <945980161e70ea98066d355bc8643ea348ac7e0a@dvhart.com>
CC: Tim Bird <e2cc6052decd123aa469ec2b69179594f915c3f9@am.sony.com>
Signed-off-by: Steven Rostedt <43232e92d70cc7aa53504ad0397085ee47bad87f@goodmis.org>

bigzz/trace-cmd,bigzz/trace-cmd