use POSIX feature macro to check for clock_gettime

note: while _POSIX_TIMERS may be "indirectly" defined by including time.h on some systems, they come from unistd.h.

tested to work on Debian and MSYS2 (gcc + clang)

fixes #345 (C part)

to be checked: Is there an environment where this does not work (it seems that on non-released mingwrt we do have the feature with clock_gettime since Dec 2017 but no posix-feature-macros at all; not sure if this is something to care for)?

Should we apply something similar to other places? See https://linux.die.net/man/7/posixoptions.

ping @Bill-Gray for review/merge

I just tried building WinGUI and WinCon, cross-compiling on my Linux box. For both, I got :

x86_64-w64-mingw32-gcc -Wl,--out-implib,pdcurses.a -static-libgcc -shared -o pdcurses.dll addch.o addchstr.o addstr.o attr.o beep.o bkgd.o border.o clear.o color.o debug.o delch.o deleteln.o getch.o getstr.o getyx.o inch.o inchstr.o initscr.o inopts.o insch.o insstr.o instr.o kernel.o keyname.o mouse.o move.o outopts.o overlay.o pad.o panel.o printw.o refresh.o scanw.o scr_dump.o scroll.o slk.o termattr.o terminfo.o touch.o util.o window.o pdcclip.o pdcdisp.o pdcgetsc.o pdckbd.o pdcscrn.o pdcsetsc.o pdcutil.o -lgdi32 -lcomdlg32 -lwinmm
/usr/bin/x86_64-w64-mingw32-ld: getch.o:getch.c:(.text+0x61e): undefined reference to `clock_gettime'
/usr/bin/x86_64-w64-mingw32-ld: getch.o:getch.c:(.text+0xe8c): undefined reference to `clock_gettime'
collect2: error: ld returned 1 exit status
make: *** [Makefile:155: pdcurses.dll] Error 1

So... compiled without trouble, but couldn't link. Presumably, there's something a little more subtle about when the various timing functions are available to us... any ideas?

I also built with Digital Mars and OpenWATCOM 1.9, each for DOS and WinCon, and it all worked and the resulting testcurs.exe files worked in DOSBox and the Wine console, respectively.

I also compiled everything with both of those compilers for OS/2. I mention this just for completeness, since I don't actually have an OS/2-capable machine on which to test the results. I got build errors with DMC, but I've had those before (don't know why); it's not related to the current issue. With OpenWATCOM, everything compiled without warnings or errors.

So I think if we can figure out a fix for the above error message, we'd be good.

For clock_gettime we'd nee to add -lrt (for glibc only necessary before 2.17, see https://www.man7.org/linux/man-pages/man3/clock_gettime.3.html - but likely no problem to do so in any case), no?

Can you please test if adding it to the Makefile(s) works for your other build environments?