build: avoid modifying `CMAKE_CXX_FLAGS`
build: avoid modifying `CMAKE_CXX_FLAGS`

This value is entirely under the control of the user.  Use
`add_compile_options` with generator expressions to apply custom flags
to the build.

apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild