minor: improve error management for missing system gcc compiler in system configuration (#260)

In the cluster configuration the file `packages.yaml` is optional

https://github.com/eth-cscs/stackinator/blob/5581430ce63f5ecfd5dae8e59b578e145b7767cf/stackinator/recipe.py#L91-L98

but at the same time there is an implicit requirement since if there is
no `gcc` specified in `packages.yaml`, we should raise an exception.


https://github.com/eth-cscs/stackinator/blob/5581430ce63f5ecfd5dae8e59b578e145b7767cf/stackinator/recipe.py#L100-L106

With this small PR, the `RuntimeError` with a nicer and a bit more
informative error message will be raised also in case no `packages.yaml`
or it is available but no `gcc` is specified.



note: I faced this many times when by mistake I ended up passing the
wrong path for the cluster configuration, and the path didn't contain
the "usual" yaml files.

Author: albestro

stackinator/recipe.py

@@ -99,7 +99,7 @@ def __init__(self, args):
 
         # extract gcc packages from system packages
         # remove gcc from packages afterwards
-        if system_packages["gcc"]:
+        if "gcc" in system_packages:
             gcc_packages = {"gcc": system_packages["gcc"]}
             del system_packages["gcc"]
         else: