support clang build

[lfeng14]

clang will trigger -Wimplicit-int, when build following code.

main() {
return 0;
}

change to

int main() {
return 0;
}

modifications in other places are also for the same reason

[keithbowes]

It's odd that clang has an issue with that. I thought that int being the default data type was standard.