CMake uses environment variables to populate options including compile and link flags. The configure step prints a few key variables.
Example: if missing cross compiler, CMake errors like:
Compiler 'arm-none-eabi-gcc' not found, you can specify search path with "PICO_TOOLCHAIN_PATH".
If the cross-compiler isn't setup correctly, errors may result like:
"arm-none-eabi-gcc: fatal error: cannot read spec file 'nosys.specs': No such file or directory"