Add Proper (Recursive) CMAKE_C_FLAGS Handling

[adfernandes]

So I had a funny problem with the standard cmake build of Bear generating executables with illegal instructions for my (admittedly) aged computer.

My base setup was Ubuntu 18.04 with the Ubuntu gcc-11 toolchain installed.

Wonder of wonders, my ancient AMD Barcelona CPU does not have the required instructions for the default codegen options of GCC 11.

I attempted to fix this by specifying CMAKE_C_FLAGS=-march=native and CMAKE_CXX_FLAGS=-march=native, but kept getting illegal instructions being generated.

So examining the CMakeLists.txt files, I found that although CMAKE_CXX_FLAGS was being respected and passed to the subprojects, CMAKE_C_FLAGS was not.

The patch below is required to properly pass build options to the subprojects.

Happily, now everything works and I can avoid buying a new computer for just a little while longer!