Environment: Fix passing envrionment variables CPPFLAGS and CFLAGS

Or other language flags that use CPPFLAGS (like CXXFLAGS). The problem
here is actually rather simple, `dict.setdefault()` doesn't work like I
thought it did, I thought it created a weak entry, but it actually is
equivalent to:
```python
if k not in dict:
    dict[k] = v
```
Instead we'll use an intermediate dictionary (a default dictionary
actually, since that makes things a little cleaner) and then add the
keys from that dict to self.options as applicable.

Test case written by Jussi, Fix by Dylan

Co-authored-by: Jussi Pakkanen
Fixes: #8361
Fixes: #8345

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