Fix build errors on MSYS platform

[carlzhc]

# make sure the required packages are already installed
$ pacman -Q | egrep '^(libbsd|ncurses|cmake)'
cmake 3.26.2-1
libbsd 0.11.7-1
ncurses 6.4-1
ncurses-devel 6.4-1

$ cd build && cmake .. && make VERBOSE=1

... ... (see the attached log file)
build.1712669255.log

[hboetes]

Thanks for your contribution. Can you show me a log with the current state of the PR?

[carlzhc]

Squashed the commits and updated the log.

[hboetes]

Thanks! I know it's working already, but wouldn't it be nice if those implicit declaration of function warnings were gone? There's probably a missing include or two in one of the header files.

[carlzhc]

Fixed those implicit declaration of function warnings.

[hboetes]

I have a feeling you might have missed the notification since I got no answer to the previous one, so where were we?

It looks good overall, but a few questions before merging:

1. The strtonum declaration in def.h needs a #if defined(__MSYS__) guard; on all other platforms it's already provided by libbsd.
2. Why does the static build need .dll appended to CURSES_LIBS? That looks a bit fragile.
3. Why does ttyio.c need sys/socket.h on MSYS specifically?