Make it possible to customize warnings

The code uses some constructs that cause compilers to spit out  warnings. As
they do no harm, one should be able to turn off such warnings and enable others
if needed.
Fix some warnings in libwww2

The directory libwww2 now compiles without any warnings with gcc and clang.
Merge pull request #7 from fuzxxl/master

Fix warnings in libwww2