Remove dependency on `thread_local`

[milleniumbug]

Aside from constexpr and thread_local, rest of the features can be easily emulated for older compilers. And even constexpr is needed just barely. With this, we can start supporting older compilers, or even MSVC.

[nabijaczleweli]

$ make seed11
clang++.exe -O3 -fomit-frame-pointer -std=c++14 -pedantic -Wall -Wextra -pipe   -isystemext/seed11/include -c -oout/build/seed11/obj/seed11_system_agnostic.o ext/seed11/src/seed11_system_agnostic.cpp
ext/seed11/src/seed11_system_agnostic.cpp:49:29: error: cannot compile this non-trivial TLS destruction yet
                        thread_local seed_device s;
                                                 ^
1 error generated.
Makefile:72: recipe for target 'out/build/seed11/obj/seed11_system_agnostic.o' failed
make: *** [out/build/seed11/obj/seed11_system_agnostic.o] Error 1

$ clang.exe --version
clang version 3.8.0 (tags/RELEASE_380/final)
Target: x86_64-w64-windows-gnu
Thread model: posix
InstalledDir: d:\Programy\Programowanie\msys2\mingw64\bin

hlep

[milleniumbug]

@nabijaczleweli You can now, as a workaround, replace the thread_local storage specifier with static everywhere it's used. You won't be able to use zero-arg make_seeded and two-arg choice in multiple threads, but other than that, it should work perfectly.