Create ability to error out if a below minimum required BLT_CXX_STD is set

[white238]

Maybe have something like BLT_MINIMUM_REQUIRED_BLT_CXX_STD:

if (BLT_CXX_STD MATCHES "^c\\+\\+([0-9]+)$" AND CMAKE_MATCH_1 VERSION_LESS BLT_MINIMUM_REQUIRED_BLT_CXX_STD)
    message(FATAL_ERROR "${PROJECT_NAME} requires BLT_CXX_STD to be c++${BLT_MINIMUM_REQUIRED_BLT_CXX_STD} or higher.")
endif()

[cyrush]

Sounds like: https://cmake.org/cmake/help/latest/prop_tgt/CXX_STANDARD_REQUIRED.html#prop_tgt:CXX_STANDARD_REQUIRED