Fix `CMAKE_CXX_FLAGS` optimize flag

[Hirozy]

According to the gcc options, here should be an uppercase O instead of a lowercase o. And the lowercase o should not appear in CMAKE_CXX_FLAGS, which will cause the file to not be output correctly when compiled.

There are some details here. https://www.rapidtables.com/code/linux/gcc/gcc-o.html https://gcc.gnu.org/onlinedocs/gcc/Overall-Options.html#Overall-Options https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html

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