[cmake] Remove duplicate command line options from build

CMAKE_CXX_FLAGS was added twice to the command line. This causes the command
line options to be doubled which works until it doesn't as not all options
can be specified twice.

For example,

clang-cl foo.c /GS- /GS- -mllvm -small-loop-cost=1 -mllvm -small-loop-cost=1
clang (LLVM option parsing): for the -small-loop-cost option: may only occur zero or one times!