The `math(EXPR)` command documents that results must fit in a 64-bit
signed integer. When built with compilers that offer the relevant
intrinsics, detect overflow and issue warnings.
Prior to commit 7c4c13ffef (math: Reject unexpected expression input
explicitly, 2018-05-18) we ignored unexpected characters in an
expression that otherwise can be parsed. In order to preserve
compatibility with projects that accidentally used this, convert the
error to a warning.