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

--- configure.ac.orig	1969-12-31 19:00:00.000000000 -0500
+++ configure.ac	2019-08-11 22:28:26.038841961 -0400
@@ -1092,9 +1092,7 @@
 solaris2*)	LOCAL_CFLAGS=-DSOLARIS ;;
 lynxos*)	LOCAL_CFLAGS=-DRECYCLES_PIDS ;;
 linux*)		LOCAL_LDFLAGS=-rdynamic		 # allow dynamic loading
-		case "`uname -r`" in
-		2.[[456789]]*|[[34]]*)	AC_DEFINE(PGRP_PIPE) ;;
-		esac ;;
+		AC_DEFINE(PGRP_PIPE) ;;
 *qnx6*)		LOCAL_CFLAGS="-Dqnx -Dqnx6" LOCAL_LIBS="-lncurses" ;;
 *qnx*)		LOCAL_CFLAGS="-Dqnx -F -3s" LOCAL_LDFLAGS="-3s" LOCAL_LIBS="-lunix -lncurses" ;;
 powerux*)	LOCAL_LIBS="-lgen" ;;
--- configure.orig	1969-12-31 19:00:00.000000000 -0500
+++ configure	2019-08-11 22:28:10.166763255 -0400
@@ -16064,10 +16064,7 @@
 solaris2*)	LOCAL_CFLAGS=-DSOLARIS ;;
 lynxos*)	LOCAL_CFLAGS=-DRECYCLES_PIDS ;;
 linux*)		LOCAL_LDFLAGS=-rdynamic		 # allow dynamic loading
-		case "`uname -r`" in
-		2.[456789]*|[34]*)	$as_echo "#define PGRP_PIPE 1" >>confdefs.h
- ;;
-		esac ;;
+		$as_echo "#define PGRP_PIPE 1" >>confdefs.h ;;
 *qnx6*)		LOCAL_CFLAGS="-Dqnx -Dqnx6" LOCAL_LIBS="-lncurses" ;;
 *qnx*)		LOCAL_CFLAGS="-Dqnx -F -3s" LOCAL_LDFLAGS="-3s" LOCAL_LIBS="-lunix -lncurses" ;;
 powerux*)	LOCAL_LIBS="-lgen" ;;