Stop checking for main function in libc

[casperisfine]

It makes very little sense and the commit that introduced this never explained why it's doing it.

This cause compilation for fail is ruby was installed with the --disable-install-static-library flag.

[heliocola]

@casperisfine : thanks for spotting this and submitting this PR.
I've looked into the definition of the have_function (and checking_message, and checking_for) and it just returns true or false, so whether it is true or false, the compilation will continue on.

I also run compile/build/test on my machine (which i used to release the last version) and all seems good.

Given this was created a while ago, I am still situating myself, I wanted to check-in if you'd expect this cause other problems.

[casperisfine]

wanted to check-in if you'd expect this cause other problems.

I don't.