2.1.2: Restore support for non-C++17 compilers

Some of the C++ changes in 2.1.1 involved a C++17 extension: 'if' initialization statements (e.g. if (bool foo = bar; bar != baz) { which meant the gem would only compile with a C++ compiler that supported that version of the language.

As users may be compiling against older versions of RE2 that only require C++11, remove this extension from the codebase to restore support for this use case.

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SHA256 checksums:

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