linux/gnu: add cmake for sample

[yf13]

This adds CMake support for linux/gnu/example_build so that to have the sample_threadx target built after the libthreadx.a target, make it easier for people to try the sample on Linux.

[fdesbiens]

Thank you for this, and apologies that it sat for so long.

The goal is right and it is still missing: the CMake build produces only the library, so trying ThreadX on Linux means using the Makefile instead. I have opened #622 to add it, derived from your two example_build/CMakeLists.txt files.

I am closing this one rather than rebasing it, and I would rather explain why than leave it looking like a rejection.

Four of the six files here have since arrived on dev by other routes: the top level's SMP selection, common_smp/CMakeLists.txt, ports/linux/gnu/CMakeLists.txt and ports_smp/linux/gnu/CMakeLists.txt. That is what the conflict is. Only the two sample targets were still missing, and those are what #622 carries.

Two details from the superseded part are worth passing back, since they may be useful to you elsewhere.

The SMP switch used if(DEFINED THREADX_SMP), which is true whenever the variable exists at all, so -DTHREADX_SMP=OFF selected the SMP build:

invocation	resulting suffix
-DTHREADX_SMP=OFF	_smp
no flag	(empty)
-DTHREADX_SMP=ON	_smp

dev now uses option() with if(THREADX_SMP), which treats OFF as off.

And the path scheme built ports_smp/${THREADX_ARCH}/... with no suffix on the architecture. That works for linux and win64, which predate the naming convention, but not for the ports named <arch>_smp. dev appends the suffix and falls back to the bare name for exactly those two.

One change was needed to your example files themselves: they read common${THREADX_SMP}/inc, which relied on THREADX_SMP being a string suffix. It is a boolean now, so that expands to commonON; #622 uses ${TX_COMMON_DIR} instead, which the top level already resolves.

Both variants now build and run — the non-SMP demo prints its banner and counters, and the SMP one prints the SMP banner. Credit for the idea and the starting point is recorded in #622 and in its commit message. Thank you for the contribution, and for #403, which was merged.