Properly append to CMAKE_C_FLAGS string

Properly append to CMAKE_C_FLAGS string

Contrary to other CMAKE variables the CMAKE_C_FLAGS variable is the
composed string of flags for the C compiler. It is therefore not a list
to append to. Current implementation results in these incorrect CFLAGS,
e.g., "-O2 -g -fblahblah;-UNDEBUG". Extending the CFLAGS this way
results in the proper CFLAGS, e.g., "-O2 -g -fblahblah -UNDEBUG".