Makefile: avoid recursively expanding expensive variables

Makefile: avoid recursively expanding expensive variables

Specifically, do not recursively expand $(CFLAGS) because this leads to
repeatedly performing compile tests (e. g. cc-disable-warning) on every
recipe execution.

Without (nproc=32):
```
$ time env -i PATH=/usr/bin BCACHEFS_FUSE=1 NO_RUST=1 make -j$(nproc)
<...>
    [LD]     bcachefs
  72,48s user 11,29s system 190% cpu 44,036 total
```

With:
```
$ time env -i PATH=/usr/bin BCACHEFS_FUSE=1 NO_RUST=1 make -j$(nproc)
<...>
    [LD]     bcachefs
  66,79s user 5,17s system 1955% cpu 3,679 total
```