Unconditionally enable PGRP_PIPE on Linux (the kernel), regardless of
the kernel version in use on the build machine.

--- configure.ac.orig	2019-01-02 09:38:44.000000000 -0500
+++ configure.ac	2019-08-15 16:40:24.271758379 -0400
@@ -1108,10 +1108,7 @@
 solaris2*)	LOCAL_CFLAGS=-DSOLARIS ;;
 lynxos*)	LOCAL_CFLAGS=-DRECYCLES_PIDS ;;
 linux*)		LOCAL_LDFLAGS=-rdynamic		 # allow dynamic loading
-		case "`uname -r`" in
-		1.*|2.[[0123]]*)	: ;;
-		*)	AC_DEFINE(PGRP_PIPE) ;;
-		esac ;;
+		AC_DEFINE(PGRP_PIPE) ;;
 netbsd*|openbsd*)	LOCAL_CFLAGS="-DDEV_FD_STAT_BROKEN" ;;
 *qnx[[67]]*)	LOCAL_LIBS="-lncurses" ;;
 *qnx*)		LOCAL_CFLAGS="-Dqnx -F -3s" LOCAL_LDFLAGS="-3s" LOCAL_LIBS="-lunix -lncurses" ;;
--- configure.orig	2019-01-02 09:43:04.000000000 -0500
+++ configure	2019-08-15 16:41:44.440155912 -0400
@@ -16312,11 +16312,7 @@
 solaris2*)	LOCAL_CFLAGS=-DSOLARIS ;;
 lynxos*)	LOCAL_CFLAGS=-DRECYCLES_PIDS ;;
 linux*)		LOCAL_LDFLAGS=-rdynamic		 # allow dynamic loading
-		case "`uname -r`" in
-		1.*|2.[0123]*)	: ;;
-		*)	$as_echo "#define PGRP_PIPE 1" >>confdefs.h
- ;;
-		esac ;;
+		$as_echo "#define PGRP_PIPE 1" >>confdefs.h ;;
 netbsd*|openbsd*)	LOCAL_CFLAGS="-DDEV_FD_STAT_BROKEN" ;;
 *qnx[67]*)	LOCAL_LIBS="-lncurses" ;;
 *qnx*)		LOCAL_CFLAGS="-Dqnx -F -3s" LOCAL_LDFLAGS="-3s" LOCAL_LIBS="-lunix -lncurses" ;;
a href='/guix/commit/.guix-authorizations?id=b4f04e0efff1fb6112b84dc6d36ea46215c336b2'>commit</a><a href='/guix/diff/.guix-authorizations?id=b4f04e0efff1fb6112b84dc6d36ea46215c336b2'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/.guix-authorizations'>
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<div class='path'>path: <a href='/guix/log/?id=b4f04e0efff1fb6112b84dc6d36ea46215c336b2'>root</a>/<a href='/guix/log/.guix-authorizations?id=b4f04e0efff1fb6112b84dc6d36ea46215c336b2'>.guix-authorizations</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/.guix-authorizations?id=b4f04e0efff1fb6112b84dc6d36ea46215c336b2&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2020-05-04 10:43:52 +0200'>2020-05-04</span></td><td><a href='/guix/commit/.guix-authorizations?id=d074f73aacc5a39aed0202d6e45721f53f34a8c0'>.guix-authorizations: Add bricewge to the committers.</a><span class='msg-avail'>...<span class='msg-tooltip'>* .guix-authorizations: Add fingerprint for bricewge.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-05-04 09:56:13 +0200'>2020-05-04</span></td><td><a href='/guix/commit/.guix-authorizations?id=bee5b7a0f8547a40f05d0891a71fff1fed4b2b3e'>.guix-authorizations: Augment.</a><span class='msg-avail'>...<span class='msg-tooltip'>* .guix-authorizations: Add all the currently authorized committers.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-05-04 09:56:12 +0200'>2020-05-04</span></td><td><a href='/guix/commit/.guix-authorizations?id=c91e27c60864faa229198f6f0caf620275c429a2'>Add '.guix-authorizations'.</a><span class='msg-avail'>...<span class='msg-tooltip'>* .guix-authorizations: New file.
</span></span></td><td>Ludovic Courtès</td></tr>
ublish.service.in (Environment): Likewise.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2018-11-23 15:42:01 +0100'>2018-11-23</span></td><td><a href='/guix/commit/etc/guix-daemon.service.in?id=e9926f80c6553fde50ce1fcfd38d6370f841efd2'>build: Binary tarball now populates the "current-guix" profile.</a><span class='msg-avail'>...<span class='msg-tooltip'>* Makefile.am (guix-binary.%.tar.xz): Pass
'--profile-name=current-guix'.  Remove glibc and glibc-utf8-locales.
* doc/guix.texi (Binary Installation): Update accordingly.
* etc/guix-install.sh
* etc/guix-install.sh (sys_create_store, sys_enable_guix_daemon)
(sys_authorize_build_farms): Likewise.
* etc/guix-publish.conf.in, etc/guix-publish.service.in,
etc/guix-daemon.conf.in, etc/guix-daemon.service.in: Update file names
accordingly.
</span></span></td><td>Ludovic Courtès</td></tr>