BLT_CXX_STD checks should work like standard cmake if

[cyrush]

I have a case where I don't want to pass an empty string to disable BLT_CXX_STD (b/c it's hard to verify empty string)

I tried IGNORE, which is one of the basic expressions that will cause cmake if statements to eval to false.

https://cmake.org/cmake/help/latest/command/if.html#basic-expressions

However since we are using property strings this option?

set_property(CACHE BLT_CXX_STD PROPERTY STRINGS c++98 c++11 c++14 c++17 c++20)

And causes a failure:

CMake Error at blt/cmake/SetupCompilerStandards.cmake:52 (message):
  "IGNORE" is an invalid entry for BLT_CXX_STD.  Valid Options are ( c++98,

Maybe we need to change the order of the calls (use set_property inside of if(BLT_CXX_STD))?