Fix some unimportant compiler warnings to have a clean build.

I ran this build:
CXXFLAGS="-Wall -Wno-c++11-long-long -pedantic -std=c++98" ./configure
make -j 1

This commit fixes the few compiler warnings that showed up (all unimportant).

There is no way to fix the issue that `long long` is not standard C++98,
but it's an extension that has been supported by most compilers for a decade,
so it's OK to ignore:
http://stackoverflow.com/questions/4017621/how-to-do-portable-64-bit-arithmetic-without-compiler-warnings