Expressions in Makefile Preprocessing
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The !IF or !ELSE IF
constantexpression consists of integer constants (in decimal or C-language notation), string constants, or commands. Use parentheses to group expressions. Expressions use C-style signed long integer arithmetic; numbers are in 32-bit two's-complement form in the range – 2147483648 to 2147483647.
Expressions can use operators that act on constant values, exit codes from commands, strings, macros, and file-system paths.