DEBUG messages are printed by CMake versions < 3.15

[jonathangjertsen]

The minimum CMake version is 3.13, but the DEBUG level for message() was added in CMake 3.15 - with earlier versions, the message is simply concatenated with DEBUG resulting in output like:

DEBUGPICO_GCC_TRIPLE defaulted to arm-none-eabi
...
DEBUGIncluding custom CMake file [redacted]/pico-sdk/src/common/pico_base/generate_config_header.cmake
DEBUGELF2UF2 will need to be built

(from PR #365)