2.4.3: Restore support for older gcc versions

Fixed

• Restore support for compiling the gem with gcc 7.3 (as used on Amazon Linux 2), fixing the "non-trivial designated initializers not supported" error introduced in version 2.4.0. Thanks to @stanhu for reporting this bug.

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SHA256 checksums:

c61f88ed98553b67b5dcf89fcce4394845b51697983b50b59c8cbff438b2f43d  re2-2.4.3-aarch64-linux.gem
12a9f7044b8667bd0b95115576d811fa22d370203b31452212f69b397b10c1c1  re2-2.4.3-arm-linux.gem
3ea5da85de0b7d8dd9a871bc939fd340c47e52925a2f7786602bb0e9b558d3b1  re2-2.4.3-arm64-darwin.gem
12a4b345e013693d5bb974d6a975f3a192d0961394986a98063a317bfaf193ad  re2-2.4.3-x64-mingw-ucrt.gem
e7fef971923684598f3cdf2efa694c54681ef07c2f332b2109f7f9249d672f0c  re2-2.4.3-x64-mingw32.gem
7783d5f18781bf9f73b27d616d10660f2604b9c0f869fe86b702393eb070d49c  re2-2.4.3-x86-linux.gem
15a3c849cea067c5d71915231541e00753c69234ac403fc497bb55ee3878591a  re2-2.4.3-x86-mingw32.gem
4e695cfe5bf6297dfd78ed02ddb4e5cff39ff045aa59d1dc59e203530b073576  re2-2.4.3-x86_64-darwin.gem
8ec7616e492881b7703cb07ed2890d7b77b0af553415bf79b19ce6ef5e5c5fed  re2-2.4.3-x86_64-linux.gem
4b38ed4b8b3b75dd4fb02d5966b44a3a0d9b9c97d6ef98d264da39dc1a0d702d  re2-2.4.3.gem