Make library more friendly towards micro controllers

Building in Buildroot for armv7m fails with:

>>> libite 2.6.2 Building
GIT_DIR=. PATH="/home/jocke/br-test-pkg/bootlin-armv7m-uclibc/host/bin:/home/jocke/br-test-pkg/bootlin-armv7m-uclibc/host/sbin:/home/jocke/go/bin:/home/jocke/gems/bin:/home/jocke/.local/bin:/home/jocke/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin:/home/jocke/.local/bin"  /usr/bin/make -j17  -C /home/jocke/br-test-pkg/bootlin-armv7m-uclibc/build/libite-2.6.2/
/usr/bin/make  all-recursive
Making all in src
  CC       libite_la-chomp.lo
  CC       libite_la-copyfile.lo
  CC       libite_la-conio.lo
  CC       libite_la-dir.lo
  CC       libite_la-erasef.lo
  CC       libite_la-fopenf.lo
  CC       libite_la-fremove.lo
  CC       libite_la-fexist.lo
  CC       libite_la-fisdir.lo
  CC       libite_la-fparseln.lo
  CC       libite_la-fsendfile.lo
  CC       libite_la-ifconfig.lo
  CC       libite_la-lfile.lo
  CC       libite_la-makepath.lo
  CC       libite_la-procval.lo
  CC       libite_la-progress.lo
  CC       libite_la-pidfile.lo
  CC       libite_la-pidfilefn.lo
  CC       libite_la-popenf.lo
  CC       libite_la-reallocarray.lo
  CC       libite_la-rsync.lo
  CC       libite_la-runbg.lo
  CC       libite_la-strlcpy.lo
  CC       libite_la-strlcat.lo
  CC       libite_la-strtonum.lo
  CC       libite_la-strmatch.lo
  CC       libite_la-systemf.lo
  CC       libite_la-strtrim.lo
  CC       libite_la-telnet.lo
  CC       libite_la-tempfile.lo
  CC       libite_la-truncatef.lo
  CC       libite_la-touchf.lo
  CC       libite_la-yorn.lo
runbg.c: In function 'runbg':
runbg.c:48:19: error: implicit declaration of function 'fork' [-Wimplicit-function-declaration]
   48 |         int pid = fork();
      |                   ^~~~

Start by reproducing the Buildroot armv7m build and inspect runbg.c at the fork() call shown in the compiler error. Check how the library's build configuration handles platforms without fork; done means the microcontroller target builds successfully without breaking supported targets.