build(cmake): use add_compile_options not C_FLAGS

[aloisklink]

CMake does not recommend modifying CMAKE_C_FLAGS directly if we can help it. Instead, we should be using add_compile_options().

This also means that we no longer overwrite CMAKE_C_FLAGS, which will mean that edgesec works better with other build systems (e.g. pdebuild).

Depends on #360 and #363 (since we no longer overwrite CMAKE_C_FLAGS, some of the debian build's C_FLAGS cause errors that need to be fixed).

[codecov]

Codecov Report

Merging #364 (99aa81d) into main (8316437) will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #364      +/-   ##
==========================================
+ Coverage   51.77%   51.80%   +0.02%     
==========================================
  Files         139      139              
  Lines       19318    19318              
==========================================
+ Hits        10002    10007       +5     
+ Misses       9316     9311       -5     

Impacted Files	Coverage Δ
tests/test_edgesec.c	94.53% <0.00%> (-0.79%)	⬇️
src/ap/ap_service.c	50.85% <0.00%> (+3.42%)	⬆️

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[mereacre]

From cmake docs:
The preprocessor definitions are added to the COMPILE_DEFINITIONS directory property for the current CMakeLists file. They are also added to the COMPILE_DEFINITIONS target property for each target in the current CMakeLists file.

In the cheri-pure branch I add compiler flags CMAKE_C_FLAGS in cheri-cross.cmake file located in CMakeToolchains. Now the question is will they be applied to the entire project?

[aloisklink]

In the cheri-pure branch I add compiler flags CMAKE_C_FLAGS in cheri-cross.cmake file located in CMakeToolchains. Now the question is will they be applied to the entire project?

Yes, CMake Toolchain files are applied globally. The add_compile_options() will only apply to targets declared after the add_compile_options() call, but this PR adds those changes before everything is declared, so that's fine.