Compile as C99

[Alcaro]

Fixes #156

The obvious fix would be renaming that variable, but adding -std=c99 instead has two advantages: It makes sure nobody else runs into similar traps later, and it improves compatibility with old compilers that default to -std=gnu89 and fail on variable declarations in for loops. (Modern GCCs default to -std=gnu11.)

Could use -std=c11 instead, but unless we're using C11 features, using C11 seems pointless.

[sm00th]

Looks good, but could you add the same reasoning to the commit message? It would make it easier to see it without having to open the browser.

I also think the variable should still be renamed to something like unixtime.

[Alcaro]

Can do

Renaming the variable seems like a separate issue to me.