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

diff -purN bash-5.0-orig/configure bash-5.0/configure
--- configure	1970-01-01 01:00:00.000000000 +0100
+++ configure	2019-09-29 11:51:42.664518665 +0200
@@ -16312,11 +16312,7 @@ solaris2.10*)	LOCAL_CFLAGS=-DSOLARIS  ;;
 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" ;;
diff -purN bash-5.0-orig/configure.ac bash-5.0/configure.ac
--- configure.ac	1970-01-01 01:00:00.000000000 +0100
+++ configure.ac	2019-09-29 11:51:10.692026225 +0200
@@ -1108,10 +1108,7 @@ solaris2.10*)	LOCAL_CFLAGS=-DSOLARIS  ;;
 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" ;;
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<tr><td><span title='2020-04-09 02:42:54 +0000'>2020-04-09</span></td><td><a href='/guix/commit/gnu/installer/record.scm?id=91c231a2223440081426929828a23c7baa0214fd'>installer: Allow Alt+Shift toggle from non-Latin keyboard layouts.</a><span class='msg-avail'>...</span></td><td>Florian Pelz</td></tr>
<tr><td><span title='2020-04-08 10:24:02 +0200'>2020-04-08</span></td><td><a href='/guix/commit/gnu/installer/record.scm?id=07a53bd512530d8f87e076263227216c467727bb'>installer: Turn help menu into parameters menu.</a><span class='msg-avail'>...</span></td><td>Mathieu Othacehe</td></tr>
<tr><td><span title='2020-04-06 15:06:03 +0200'>2020-04-06</span></td><td><a href='/guix/commit/gnu/installer/record.scm?id=786c9c39bc0826737e99f0c750ea30033cdbc202'>installer: Add a help page.</a><span class='msg-avail'>...</span></td><td>Mathieu Othacehe</td></tr>
<tr><td><span title='2019-01-17 14:04:24 +0100'>2019-01-17</span></td><td><a href='/guix/commit/gnu/installer/record.scm?id=69a934f23ae1bd7dda9ec269a6ce3012e13c9011'>installer: Add partitioning support.</a><span class='msg-avail'>...</span></td><td>Mathieu Othacehe</td></tr>
<tr><td><span title='2019-01-17 14:04:23 +0100'>2019-01-17</span></td><td><a href='/guix/commit/gnu/installer/record.scm?id=b51bde71a9385f4e81fbea258bfb9e8ff48be119'>installer: Add services page.</a><span class='msg-avail'>...</span></td><td>Mathieu Othacehe</td></tr>
<tr><td><span title='2019-01-17 14:04:23 +0100'>2019-01-17</span></td><td><a href='/guix/commit/gnu/installer/record.scm?id=c088b2e47f6675199f1ef545df7d04d4532e64e3'>installer: Do not ask for keyboard model.</a><span class='msg-avail'>...</span></td><td>Mathieu Othacehe</td></tr>
<tr><td><span title='2019-01-17 14:04:22 +0100'>2019-01-17</span></td><td><a href='/guix/commit/gnu/installer/record.scm?id=dc5f3275ecbddc804875899e9e457299a835d7ab'>installer: Add configuration formatter.</a><span class='msg-avail'>...</span></td><td>Mathieu Othacehe</td></tr>
<tr><td><span title='2019-01-17 14:04:21 +0100'>2019-01-17</span></td><td><a href='/guix/commit/gnu/installer/record.scm?id=a49d633c0c65975263270f5ac0050482ca6a5513'>installer: Move everything to the build side.</a><span class='msg-avail'>...</span></td><td>Mathieu Othacehe</td></tr>