fix(build): Use space-separated string for CMAKE_REQUIRED_FLAGS This is a cherry-pick of commit d733c11206a350f2af3cd13c6d5929e9fb6b27a1 from branch 'libpng18'. Co-authored-by: Brad King <brad.king@kitware.com> Signed-off-by: Cosmin Truta <ctruta@gmail.com>