cmake: Add GCC -Og flag fallback to -O0.

[stephanosio]

The -Og (optimise for debugging) flag is only available for GCC 4.8.0
and above, and specifying it for a GCC version lower than 4.8.0 will
result in a compilation error.

This commit adds a run-time check for compiler -Og optimisation flag
support and a fallback to -O0 (disable optimisation) when -Og flag is
unsupported.

Signed-off-by: Stephanos Ioannidis [email protected]

[SebastianBoe]

gcc 4.8 was released 6 years ago.

Let's not add build overhead for all users due to a small amount of users having outdated toolchains.

Erroring-out as we are now doing when the user explicitly (OPTIMIZE_FOR_DEBUG_FLAG is not the default) enables a feature they can't have is a sane policy.

A user that is affected can either enable CONFIG_NO_OPTIMIZATIONS instead of CONFIG_DEBUG_OPTIMIZATIONS, or invoke CMake with -DOPTIMIZE_FOR_DEBUG_FLAG=-O0 .

And IMHO this is more than flexible enough.

[stephanosio]

Let's not add build overhead for all users due to a small amount of users having outdated toolchains.

well, I just happened to be one of those users (I am stuck with an ancient toolchain because the vendor of this proprietary arch I am currently running Zephyr on refuses to release modern toolchain), but then I suppose check_compiler_flag is not free by any means.

@SebastianBoe If check_compiler_flag is too much of a burden, would you be okay with just a version check using CMAKE_C_COMPILER_VERSION because we know that GCC 4.8.0 is the first version to support -Og?

[SebastianBoe]

A version check is OK.