When `toolchain_is_ok` fails, the error message points the user at the
CMake logs. But those logs will be empty if the user tried to compile
more than once (typically: cleans everything and tries again). So tell
the user that cleaning the toolchain cache is required to see the error.
Tell the user to "move" the cache instead of removing it in case
technical support needs the cache for forensics.
Some finicky toolchains can be "non-deterministic" and fail
_sometimes_. For instance a license server can be flaky, or the
toolchain can require an "elaborate" set of environment variables
triggering some configuration "trial-and-error". In such a
non-deterministic case deleting the cache is enough to get rid of the
issue and move on! Looking at logs is not even required; even
better. Once the toolchain cache believes that the toolchain works, any
future toolchain glitch will be obvious at actual compilation time.
To test all this:
```
# Verify that the toolchain can compile a dummy file, if it is not we
# won't be able to test for compatibility with certain C flags.
-zephyr_check_compiler_flag(C "" toolchain_is_ok)
+zephyr_check_compiler_flag(C "-fubar" toolchain_is_ok)
assert(toolchain_is_ok "The toolchain is unable to build a dummy C file.\
```
Signed-off-by: Marc Herbert <marc.herbert@intel.com>
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