One check for sys/un.h ought to be enough. Zero checks for sys/shm.h are not.

configure

@@ -2552,7 +2552,7 @@ mkinstalldirs="\$(SHELL) \$(top_srcdir)/config/mkinstalldirs"
 
 
 
-for ac_prog in mawk gawk nawk awk
+for ac_prog in gawk mawk nawk awk
 do
 # Extract the first word of "$ac_prog", so it can be a program name with args.
 set dummy $ac_prog; ac_word=$2
@@ -4298,7 +4298,7 @@ fi
 ##
 ## Header files
 ##
-for ac_hdr in crypt.h dld.h endian.h fp_class.h getopt.h ieeefp.h netinet/tcp.h pwd.h sys/ipc.h sys/pstat.h sys/select.h sys/sem.h sys/socket.h sys/un.h sys/types.h sys/un.h termios.h kernel/OS.h kernel/image.h SupportDefs.h
+for ac_hdr in crypt.h dld.h endian.h fp_class.h getopt.h ieeefp.h netinet/tcp.h pwd.h sys/ipc.h sys/pstat.h sys/select.h sys/sem.h sys/socket.h sys/shm.h sys/types.h sys/un.h termios.h kernel/OS.h kernel/image.h SupportDefs.h
 do
 ac_safe=`echo "$ac_hdr" | sed 'y%./+-%__p_%'`
 echo $ac_n "checking for $ac_hdr""... $ac_c" 1>&6

configure.in

@@ -660,7 +660,7 @@ fi
 ## Header files
 ##
 dnl sys/socket.h and sys/types.h are required by AC_FUNC_ACCEPT_ARGTYPES
-AC_CHECK_HEADERS([crypt.h dld.h endian.h fp_class.h getopt.h ieeefp.h netinet/tcp.h pwd.h sys/ipc.h sys/pstat.h sys/select.h sys/sem.h sys/socket.h sys/un.h sys/types.h sys/un.h termios.h kernel/OS.h kernel/image.h SupportDefs.h])
+AC_CHECK_HEADERS([crypt.h dld.h endian.h fp_class.h getopt.h ieeefp.h netinet/tcp.h pwd.h sys/ipc.h sys/pstat.h sys/select.h sys/sem.h sys/socket.h sys/shm.h sys/types.h sys/un.h termios.h kernel/OS.h kernel/image.h SupportDefs.h])
 
 AC_CHECK_HEADERS([readline/readline.h readline.h], [break])
 AC_CHECK_HEADERS([readline/history.h history.h], [break])