Even more gcc warning options

[jbytheway]

Summary

SUMMARY: None

Purpose of change

More compiler checks.

Describe the solution

Adding:

• -Wformat-signedness which looks for e.g. printf("%d", n); where n is unsigned. There was one example of this in the tests.
• -Wlogical-op which looks for e.g. a & b which should probably have been a && b. No hits on this one, but it seems like a good warning to have.
• -Wnon-virtual-dtor which looks for classes with virtual functions but non-virtual destructors. Deleting pointers of such type can be UB. There were a few cases; I'm not sure whether any of them were actually being used incorrectly, but unless there's an extreme performance requirement then we should play it safe.
• -Wredundant-decls which looks for duplicate function declarations. There were a couple of these which essentially amounted to functions being declared rather than the proper header being included.
• -Wrestrict which looks for violations of the restrict keyword. That only exists in C++ as an extension, I think. In any case, no examples highlighted. (turned out not to be supported by gcc 5.3)
• -Wunused-macros which looks for unused macros declared in cpp files (of course, it can't know whether macros in headers are unused. It found a few such.
• -Wzero-as-null-pointer-constant which is another way to encourage use of nullptr. There was one example, in the curses build where clang-tidy hasn't already eliminated all NULLs.

Describe alternatives you've considered

Adding even more warnings!

Testing

Compiled and ran unit tests.

Additional context

There's a good chance that one or both of the Mingw and Android builds will fail, so don't merge until Travis has shown them to be clean.

[kevingranade]

src/game.cpp:6841:9: error: macro is not used [-Werror,-Wunused-macros]

#define MAXIMUM_ZOOM_LEVEL 4

    ^

[ZhilkinSerg]

 In file included from tools/format/format.cpp:3:0:
tools/format/getpost.h: In function ‘std::__cxx11::string urlDecode(std::__cxx11::string)’:
tools/format/getpost.h:47:67: error: zero as null pointer constant [-Werror=zero-as-null-pointer-constant]
                 tmpchar = static_cast<char>( strtol( tmp, NULL, 0 ) );
                                                                   ^