Comment 4 for bug 913369

With the merge in #6394, the huge majority of these warnings have now been killed!

Attached is the remaining 93 lines of warning output. (Including the lines only containing a caret pointing at the issue).

These ones in particular are bugs that need fixing:

src/md5.h:94:7: warning: 'MD5Checksum<basic_FileRead<StreamRead> >::Data' hides overloaded virtual function [-Woverloaded-virtual]
src/ui_basic/checkbox.cc:145:23: warning: & has lower precedence than !=; != will be evaluated first [-Wparentheses]
    if (oldhl != m_flags & Is_Highlighted)
src/logic/player.cc:1078:22: warning: variable 'id' is uninitialized when used here [-Wuninitialized]
    for (uint32_t id; id < stocks.size(); ++id) {

It would be nice to also get rid of the unused variable warnings, for this it may be necessary to add an UNUSED() macro to declare intention that a variable should be left unused. Or alternatively to rewrite loops which don't use the automatically declared variable to just iterate using a for(i = 0; i < n_things; i++) loop instead.