configure.ac is out of sync, some gcc checks are missing

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configure.ac is out of sync, the gcc newer than 4.8 and gcc 10 checks are missing. They have only been added to configure, but not to configure.ac. When autoconf is used, the resulting configure script does not support gcc 10 and newer.

The related changes are part of commit https://github.com/chenall/grub4dos/commit/8c4bc2dacacf72ff86babd26d77e7b7d5faa6295