Under gcc-9 (e.g. used by Ubuntu 20.04), compilation fails with:
```
/usr/include/c++/9/variant: In instantiation of ‘constexpr const size_t std::variant_size_v<user_var>’:
/usr/include/c++/9/variant:1023:10: required from ‘struct std::__detail::__variant::__gen_vtable<true, void, user_var::operator++(int)::<lambda(auto:22&&)>&&, user_var&>’
/usr/include/c++/9/variant:1656:23: required from ‘constexpr decltype(auto) std::__do_visit(_Visitor&&, _Variants&& ...) [with bool __use_index = false; bool __same_return_types = true; _Visitor = user_var::operator++(int)::<lambda(auto:22&&)>; _Variants = {user_var&}]’
/usr/include/c++/9/variant:1672:29: required from ‘constexpr std::__detail::__variant::__visit_result_t<_Visitor, _Variants ...> std::visit(_Visitor&&, _Variants&& ...) [with _Visitor = user_var::operator++(int)::<lambda(auto:22&&)>; _Variants = {user_var&}; std::__detail::__variant::__visit_result_t<_Visitor, _Variants ...> = void]’
scripts/DallasFuncs.h:21:75: required from here
/usr/include/c++/9/variant:94:29: error: incomplete type ‘std::variant_size<user_var>’ used in nested name specifier
94 | inline constexpr size_t variant_size_v = variant_size<_Variant>::value;
```
Judging from the date of P2162, calling std::visit with std::variant
derivatives is only specified for C++20 and onwards. But Descent3
only asks for C++17.
Since ``class user_var`` does not have any members that need would
need to be accessed via std::visit, we can "add a hint" and
explicitly specify the base type.
$GIT/scripts/LEVEL15.cpp: In function ‘void aMatCenPuzzleInit()’:
$GIT/scripts/LEVEL15.cpp:833:38: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
833 | #define MagicMatCenSwitchSequence (*((int *)(&User_vars[17])))
$GIT/scripts/LEVEL15.cpp:834:25: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
834 | #define MatCenStateA (*((int *)(&User_vars[0])))
...
$GIT/scripts/Level6.cpp: In function ‘void aPriestKeyEnter(int)’:
$GIT/scripts/Level6.cpp:910:47: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
910 | #define Var_ThereIsPlayerInPriestKeyPuzzle (*((int *)(&User_vars[7])))
Turn ``User_var`` into an array of std::variant, the latter of which can hold
either float or int. Savegames do not carry the necessary type information
which variant (float/int) is in use; instead, this is statically decided by
the level DLL logic on a per-index basis. This approach is retained for now.
A lot of ``Var_something = 0`` is used despite Var_something being
logically used as float, so we need to override op= to keep the
variant type as-is.
Due to ``#include "DallasFuncs.cpp"``, DF is recompiled 52 times.
Rework it to build just once. The compile time goes down for me
from 1m45.3s to 1m38.8s on my 1135G7 CPU running make -j8.
