build generate_constants.cpp with WIN32_LEAN_AND_MEAN (#269)
build generate_constants.cpp with WIN32_LEAN_AND_MEAN (#269)

This fixes a broken build due to BoringSSL types conflicting with Windows. Using this define so that fewer windows includes are brought in.
google/conscrypt,google/conscrypt,google/conscrypt,google/conscrypt,google/conscrypt,google/conscrypt