toolchain/52806: libiberty build fails on Cygwin due to process.h not being available in the search path

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>Number:         52806
>Category:       toolchain
>Synopsis:       libiberty build fails on Cygwin due to process.h not being available in the search path
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    toolchain-manager
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sun Dec 10 14:30:00 +0000 2017
>Originator:     Sevan Janiyan
>Release:        NetBSD-8
>Organization:
>Environment:
CYGWIN_NT-6.1 X61s 2.9.0(0.318/5/3) 2017-09-12 10:18 x86_64 Cygwin
>Description:
Attempting to build an amd64 release on Cygwin fails when building components from libiberty. The configure stage detects the availability of process.h (Cygwin specific header) and sets HAVE_PROCESS_H but the build process fails due to the header not being in its search path.

#    create  liblto_plugin/pex-unix.d
CC=/cygdrive/c/Users/Me/Downloads/src/obj/tooldir.CYGWIN_NT-6.1-2.9.0-x86_64/bin/x86_64--netbsd-gcc /cygdrive/c/Users/Me/Downloads/src/obj/tooldir.CYGWIN_NT-6.1-2.9.0-x86_64/bin/nbmkdep -f pex-unix.d.tmp  --   -std=gnu99   --sysroot=/cygdrive/c/Users/Me/Downloads/src/obj/destdir.amd64 -I/cygdrive/c/Users/Me/Downloads/src/external/gpl3/gcc/dist/include -I. -DHAVE_CONFIG_H -Dxstrdup=strdup -Dxexit=exit -Dxcalloc=calloc -Dxmalloc=malloc -Dxrealloc=realloc     /cygdrive/c/Users/Me/Downloads/src/external/gpl3/gcc/dist/libiberty/pex-unix.c &&  mv pex-unix.d.tmp pex-unix.d
--- dependall-libexec ---
--- dependall ---
--- dependall-external ---
/cygdrive/c/Users/Me/Downloads/src/external/gpl3/gcc/dist/libiberty/pex-unix.c:59:21: fatal error: process.h: No such file or directory
compilation terminated.
nbmkdep: compile failed.
*** [pex-unix.d] Error code 1

nbmake[9]: stopped in /cygdrive/c/Users/Me/Downloads/src/external/gpl3/gcc/lib/liblto_plugin
1 error


If I remove the check for process.h and rebuild, it fails because spawnve() and such are not found (because they're prototyped in process.h) 
>How-To-Repeat:
./build.sh -U -m amd64 release
>Fix: