Mathematical expressions.
math(EXPR <output-variable> <math-expression>)
EXPR evaluates mathematical expression and returns result in the output variable. Example mathematical expression is 5 * (10 + 13). Supported operators are +, -, *, /, %, |, &, ^, ~, <<, >>, and (...). They have the same meaning as they do in C code.
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https://cmake.org/cmake/help/v3.7/command/math.html