cmake: Fix SSP compiler flag check
cmake: Fix SSP compiler flag check

Need to use CMAKE_REQUIRED_FLAGS when checking SSP, not
CMAKE_REQUIRED_DEFINITIONS.

Signed-off-by: Joel Carlson <d7b603241b8a42129a6a698ae087a34c06f1e1a1@gmail.com>
Reviewed-by: Andreas Schneider <5be00ddc76278cf6077f5047ca3384a88460c671@cryptomilk.org>

clibs/cmocka,clibs/cmocka