gmtlogo: Create the gmt_letters array dynamically to avoid MSVC compiler error

[seisman]

As reported in conda-forge/gmt-feedstock#302 (comment), building GMT source code with MSVC started to fail with following errors:

%SRC_DIR%\src\gmtlogo.c(38): error C2099: initializer is not a constant
%SRC_DIR%\src\gmtlogo.c(131): error C2099: initializer is not a constant
%SRC_DIR%\src\gmtlogo.c(145): error C2099: initializer is not a constant

I believe it's caused by behavior changes of new MSVC versions. In PR #8627 and #8628, we tried to improve the handling of NAN on Windows to avoid the error, but it seems we don't have a good solution there.

Instead, we can refactor the gmt_letters array in gmtlogo.c to avoid initializing gmt_letters with NAN. This is done by splitting the gmt_letters array into three separate arrays (for the letter G, M and T, respectively), and then creating the gmt_letters array by joining the three arrays with NAN records. This PR also reverts the changes in PR #8627

Now the build passes.

[seisman]

The failures on macOS is unrelated and can be ignored.

[joa-quim]

We now have some warnings that I don't fully understand. If it works a simpler solution would have been to do

const double d_nan = NAN;

and use d_nan instead of NAN

[seisman]

I don't see any related warnings in the CI jobs, on both Linux and Windows. What are exactly the warnings?

[joa-quim]

Hmm, I don't see them anymore. It was something about possible overflows.
I also see these for sometime (but they are new) that I don't understand.

D:\a\gmt\gmt\src\gmt_init.c(3754): warning C4129: 'g': unrecognized character escape sequence
D:\a\gmt\gmt\src\gmt_init.c(3755): warning C4129: 'g': unrecognized character escape sequence

[seisman]

Hmm, I don't see them anymore. It was something about possible overflows.

Yes, it's caused by typos and has been fixed in cd21d88.

I also see these for sometime (but they are new) that I don't understand.

D:\a\gmt\gmt\src\gmt_init.c(3754): warning C4129: 'g': unrecognized character escape sequence
D:\a\gmt\gmt\src\gmt_init.c(3755): warning C4129: 'g': unrecognized character escape sequence

I also see these warnings, but they're unrelated to changes in this PR.