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/gnu/packages/lua.scm?id=1589a825daf7d994a7236fdffdc5a2f98fe8cc38'>commit</a><a href='/guix/diff/gnu/packages/lua.scm?id=1589a825daf7d994a7236fdffdc5a2f98fe8cc38'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/gnu/packages/lua.scm'>
<input type='hidden' name='id' value='1589a825daf7d994a7236fdffdc5a2f98fe8cc38'/><select name='qt'>
<option value='grep'>log msg</option>
<option value='author'>author</option>
<option value='committer'>committer</option>
<option value='range'>range</option>
</select>
<input class='txt' type='search' size='10' name='q' value=''/>
<input type='submit' value='search'/>
</form>
</td></tr></table>
<div class='path'>path: <a href='/guix/log/?id=1589a825daf7d994a7236fdffdc5a2f98fe8cc38'>root</a>/<a href='/guix/log/gnu?id=1589a825daf7d994a7236fdffdc5a2f98fe8cc38'>gnu</a>/<a href='/guix/log/gnu/packages?id=1589a825daf7d994a7236fdffdc5a2f98fe8cc38'>packages</a>/<a href='/guix/log/gnu/packages/lua.scm?id=1589a825daf7d994a7236fdffdc5a2f98fe8cc38'>lua.scm</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/gnu/packages/lua.scm?id=1589a825daf7d994a7236fdffdc5a2f98fe8cc38&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2022-09-19 10:25:05 +0300'>2022-09-19</span></td><td><a href='/guix/commit/gnu/packages/lua.scm?id=c46a96fe33a62f58ec38754e7d4829103bb6948b'>gnu: luajit: Remove riscv64-linux from supported-systems.</a><span class='msg-avail'>...</span></td><td>Efraim Flashner</td></tr>
<tr><td><span title='2022-08-29 09:55:44 +0300'>2022-08-29</span></td><td><a href='/guix/commit/gnu/packages/lua.scm?id=75d73172904f3b6a0fc077297b5a00750af3b16c'>gnu: fennel: Update to 1.2.0.</a><span class='msg-avail'>...</span></td><td>Andrew Tropin</td></tr>
<tr><td><span title='2022-05-18 22:29:55 +0200'>2022-05-18</span></td><td><a href='/guix/commit/gnu/packages/lua.scm?id=2db523a4a8c483f64e2f02bf1aba770892c9aaf0'>gnu: lua-luv: Use gexps.</a><span class='msg-avail'>...</span></td><td>Luis Henrique Gomes Higino</td></tr>
<tr><td><span title='2022-05-18 22:29:55 +0200'>2022-05-18</span></td><td><a href='/guix/commit/gnu/packages/lua.scm?id=42772a2fa7bba87168d8e9e6803d3152b4899acd'>gnu: lua-luv: Update to 1.43.0-0.</a><span class='msg-avail'>...</span></td><td>Luis Henrique Gomes Higino</td></tr>
<tr><td><span title='2022-01-26 17:54:58 +0100'>2022-01-26</span></td><td><a href='/guix/commit/gnu/packages/lua.scm?id=73d775f518e8bf9c658cb0a161b22209c71e75aa'>gnu: fennel: Update to 03c1c95.</a><span class='msg-avail'>...</span></td><td>Brandon Lucas</td></tr>
<tr><td><span title='2022-01-26 17:54:58 +0100'>2022-01-26</span></td><td><a href='/guix/commit/gnu/packages/lua.scm?id=532153d49cec44c1e010938fd288dd6c44509487'>gnu: Add fnlfmt.</a><span class='msg-avail'>...</span></td><td>Brandon Lucas</td></tr>
<tr><td><span title='2022-01-24 09:12:59 -0800'>2022-01-24</span></td><td><a href='/guix/commit/gnu/packages/lua.scm?id=ecb3f1b96a5895999cd800187b2197a78b56760a'>gnu: luajit: Remove powerpc64le-linux from supported-systems.</a><span class='msg-avail'>...</span></td><td>Chris Marusich</td></tr>
<tr><td><span title='2022-01-13 23:11:20 +0100'>2022-01-13</span></td><td><a href='/guix/commit/gnu/packages/lua.scm?id=2c9902cae9b927726e4a35ada9f079e69e91dabe'>gnu: lua: Add version 5.4.3.</a><span class='msg-avail'>...</span></td><td>Alex Devaure</td></tr>
<tr><td><span title='2021-12-17 19:07:17 +0100'>2021-12-17</span></td><td><a href='/guix/commit/gnu/packages/lua.scm?id=e661f4eff425e8f914fef8d76690aa0567ad765e'>gnu: Remove unneeded uses of custom GCC versions.</a><span class='msg-avail'>...</span></td><td>Marius Bakke</td></tr>
<tr><td><span title='2021-12-13 17:48:25 +0100'>2021-12-13</span></td><td><a href='/guix/commit/gnu/packages/lua.scm?id=8394619baceb118df92e355377fd543bb1aa501a'>gnu: Simplify package inputs.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2021-11-17 23:43:14 +0100'>2021-11-17</span></td><td><a href='/guix/commit/gnu/packages/lua.scm?id=3a317f7476f8c6012e166ff9f340f861938721c9'>Merge branch 'master' into core-updates-frozen</a></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2021-11-16 21:04:58 +0200'>2021-11-16</span></td><td><a href='/guix/commit/gnu/packages/lua.scm?id=6ed2c5e2a414181af88793dc01d17e16914fb0dd'>gnu: fennel: Update to 1.0.0.</a><span class='msg-avail'>...</span></td><td>jgart</td></tr>
<tr><td><span title='2021-11-14 00:10:58 +0100'>2021-11-14</span></td><td><a href='/guix/commit/gnu/packages/lua.scm?id=459d7cfbc4f4821b645e93149717d38b7a80135e'>gnu: fmt: Update to 8.0.1.</a><span class='msg-avail'>...</span></td><td>Greg Hogan</td></tr>