Running 'gcc --version' for every image recipe is slow and increases parsing
time/resource usage for no good reason. Only compute the value in when we're
really running the task/function.
(From OE-Core rev: bf49316bb9913b7c89de64d6a194be31aa66e16b)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>