cmake: remove code that manipulates CMAKE_C_FLAGS
cmake: remove code that manipulates CMAKE_C_FLAGS

Manipulating CMAKE_C_FLAGS directly within the CMakeLists.txt is
generally frowned upon, and error-prone as compilers evolve.

Signed-off-by: GitHub <1505422b2465e9a84f6fdfaa161078890c593f06@github.com>

alanxz/rabbitmq-c,alanxz/rabbitmq-c,alanxz/rabbitmq-c