fheroes2: drop `FHEROES2_STRICT_COMPILATION=1`

The only effect `FHEROES2_STRICT_COMPILATION=1` on `fheroes2`
is to add `-Werror` to the compiler flags. Unfortunately
blanket `-Werror` usually breaks the builds with every major
compiler update.

For example right now `fheroes2` currently fails on `gcc-16`
from `gcc` `master` as:

```
fheroes2> In file included from /nix/store/nq52d716cprkic7kvb68izz590lyznjf-gcc-16.0.0.99999999/include/c++/16.0.0.99999999/memory:80,
fheroes2>                  from ../../fheroes2/gui/ui_button.h:26,
fheroes2>                  from ../../fheroes2/gui/ui_button.cpp:21:
fheroes2> In member function 'std::__uniq_ptr_impl<_Tp, _Dp>::pointer std::__uniq_ptr_impl<_Tp, _Dp>::_M_ptr() const [with _Tp = unsigned char; _Dp = std::default_delete<unsigned char []>]',
fheroes2>     inlined from 'std::unique_ptr<_Tp [], _Dp>::pointer std::unique_ptr<_Tp [], _Dp>::get() const [with _Tp = unsigned char; _Dp = std::default_delete<unsigned char []>]' at /nix/store/nq52d716cprkic7kvb68izz590lyznjf-gcc-16.0.0.99999999/include/c++/16.0.0.99999999/bits/unique_ptr.h:742:27,
fheroes2>     inlined from 'std::unique_ptr<_Tp [], _Dp>::operator bool() const [with _Tp = unsigned char; _Dp = std::default_delete<unsigned char []>]' at /nix/store/nq52d716cprkic7kvb68izz590lyznjf-gcc-16.0.0.99999999/include/c++/16.0.0.99999999/bits/unique_ptr.h:759:19,
fheroes2>     inlined from 'bool fheroes2::Image::empty() const' at ../../engine/image.h:86:21,
fheroes2>     inlined from 'virtual const fheroes2::Sprite& fheroes2::ButtonSprite::_getDisabled() const' at ../../fheroes2/gui/ui_button.cpp:524:29,
fheroes2>     inlined from 'void fheroes2::ButtonBase::_updateReleasedArea()' at ../../fheroes2/gui/ui_button.h:171:82,
fheroes2>     inlined from 'void fheroes2::ButtonBase::_updateButtonAreas()' at ../../fheroes2/gui/ui_button.h:149:32,
fheroes2>     inlined from 'fheroes2::Button::Button(int32_t, int32_t, int, uint32_t, uint32_t)' at ../../fheroes2/gui/ui_button.h:194:31,
fheroes2>     inlined from 'std::__detail::__unique_ptr_t<_Tp> std::make_unique(_Args&& ...) [with _Tp = fheroes2::Button; _Args = {const int&, const int&, const int&, const unsigned int&, const unsigned int&}]' at /nix/store/nq52d716cprkic7kvb68izz590lyznjf-gcc-16.0.0.99999999/include/c++/16.0.0.99999999/bits/unique_ptr.h:1086:30,
fheroes2>     inlined from 'void fheroes2::ButtonGroup::createButton(int32_t, int32_t, int, uint32_t, uint32_t, int)' at ../../fheroes2/gui/ui_button.cpp:626:52:
fheroes2> /nix/store/nq52d716cprkic7kvb68izz590lyznjf-gcc-16.0.0.99999999/include/c++/16.0.0.99999999/bits/unique_ptr.h:192:67: error: array subscript 22 is outside array bounds of 'void [96]' [-Werror=array-bounds=]
fheroes2>   192 |       pointer    _M_ptr() const noexcept { return std::get<0>(_M_t); }
fheroes2>       |                                                                   ^
fheroes2> In function 'std::__detail::__unique_ptr_t<_Tp> std::make_unique(_Args&& ...) [with _Tp = fheroes2::Button; _Args = {const int&, const int&, const int&, const unsigned int&, const unsigned int&}]',
fheroes2>     inlined from 'void fheroes2::ButtonGroup::createButton(int32_t, int32_t, int, uint32_t, uint32_t, int)' at ../../fheroes2/gui/ui_button.cpp:626:52:
fheroes2> /nix/store/nq52d716cprkic7kvb68izz590lyznjf-gcc-16.0.0.99999999/include/c++/16.0.0.99999999/bits/unique_ptr.h:1086:30: note: at offset 176 into object of size 96 allocated by 'operator new'
fheroes2>  1086 |     { return unique_ptr<_Tp>(new _Tp(std::forward<_Args>(__args)...)); }
fheroes2>       |                              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```

While these errors are useful for developers they
usualllly do more harm than good for distributions.