New utility macro for conditionally setting a variable?

It might be useful to add a new utility macro to set a variable using a ternary operator

blt_set(TO <var>
        IF     <cnd>
        THEN   <true_val>
        ELSE   <false_val>
       [CACHE  TRUE|FALSE] 
        )

This would replace the following

if( <cnd> )
   set(<var> <true_val>)
else()
   set(<var> <false_val>)
endif()

Other possible names:

• blt_conditional_set
• blt_ternary_set
• blt_set_if
• blt_ternary

The issue names no implementation file, test, or entry point. Start by locating BLT's existing utility macros and CMake tests, then compare their argument and CACHE conventions with the proposed conditional form; done means agreeing on the macro name and behavior and adding coverage for both condition branches and cache handling.