Log Message:
tomlplusplus: update to 3.4.0.

## v3.4.0

#### Fixes

-   fixed `value_flags` not being preserved correctly when inserting into tables \ 
and arrays (#108) (@LebJe)
-   fixed `toml::value::flags()` not being cleared when `std::move`-ing a value
-   fixed error in README (#195) (@andrewkcorcoran)
-   fixed compiler error when using NVCC (#198) (@thompsonnoahe)
-   fixed `noexcept(...)` sometimes being incorrectly derived on `for_each()`
-   fixed `for_each()` compilation error on GCC <= 7 (#197) \ 
(@sagi-ottopia, @damirbarr)
-   fixed `FLT_RADIX` check getting broken by Intel MKL headers (#202) (@iago-lito)
-   fixed keys containing `\t` incorrectly formatting as bare keys \ 
(@jasmine-zhu, @arp242)
-   fixed keys containing `\t` and `\n` not round-tripping correctly (@arp242)

#### Additions

-   added support for using enums with `value_or()`

#### Changes:

-   renamed header files to have `.hpp` extension (`toml.h` is still present for \ 
backwards-compatibility)

#### Build system:

-   fixed meson builds with `-Ddefault_library=static` having hidden symbols on \ 
GNU compilers (#201) (@vlad0x00)