Fix build failure on systems without spawn.h

9 jobs for <span class="ref-name">non-POSIX-spawn</span> in 18 minutes and 47 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #113593
cross-android_api21_arm64

00:01:36

passed #113594
cross-android_api28_arm64

00:01:35

passed #113595
cross-mingw64

00:02:14

passed #113592
fedora-autotools-x86_64

00:05:44

passed #113591
fedora-x86_64

00:05:27

manual #113598
freebsd-11 allowed to fail manual
freebsd-11-x86_64
passed #113596
win32
msys2-mingw32

00:10:48

passed #113597
win32
vs2017-x64

00:07:50

 
  Coverage
passed #113599
coverage

00:02:05

74.6%