libtool.m4: fix nm BSD flag detection

Libtool needs to get BSD-format (or MS-format) output out of the system
nm, so that it can scan generated object files for symbol names for
-export-symbols-regex support.  Some nms need specific flags to turn on
BSD-formatted output, so libtool checks for this in its AC_PATH_NM.
Unfortunately the code to do this has a pair of interlocking flaws:

 - it runs the test by doing an nm of /dev/null.  Some platforms
   reasonably refuse to do an nm on a device file, but before now this
   has only been worked around by assuming that the error message has a
   specific textual form emitted by Tru64 nm, and that getting this
   error means this is Tru64 nm and that nm -B would work to produce
   BSD-format output, even though the test never actually got anything
   but an error message out of nm -B.  This is fixable by nm'ing *nm
   itself* (since we necessarily have a path to it).

 - the test is entirely skipped if NM is set in the environment, on the
   grounds that the user has overridden the test: but the user cannot
   reasonably be expected to know that libtool wants not only nm but
   also flags forcing BSD-format output.  Worse yet, one such "user" is
   the top-level Cygnus configure script, which neither tests for
   nor specifies any BSD-format flags.  So platforms needing BSD-format
   flags always fail to set them when run in a Cygnus tree, breaking
   -export-symbols-regex on such platforms.  Libtool also needs to
   augment $LD on some platforms, but this is done unconditionally,
   augmenting whatever the user specified: the nm check should do the
   same.

   One wrinkle: if the user has overridden $NM, a path might have been
   provided: so we use the user-specified path if there was one, and
   otherwise do the path search as usual.  (If the nm specified doesn't
   work, this might lead to a few extra pointless path searches -- but
   the test is going to fail anyway, so that's not a problem.)

(Tested with NM unset, and set to nm, /usr/bin/nm, my-nm where my-nm is a
symlink to /usr/bin/nm on the PATH, and /not-on-the-path/my-nm where
*that* is a symlink to /usr/bin/nm.)

ChangeLog
2021-09-27  Nick Alcock  <nick.alcock@oracle.com>

	PR libctf/27967
	* libtool.m4 (LT_PATH_NM): Try BSDization flags with a user-provided
	NM, if there is one.  Run nm on itself, not on /dev/null, to avoid
	errors from nms that refuse to work on non-regular files.  Remove
	other workarounds for this problem.  Strip out blank lines from the
	nm output.

ChangeLog

@@ -1,3 +1,12 @@
+2021-09-27  Nick Alcock  <nick.alcock@oracle.com>
+
+	PR libctf/27967
+	* libtool.m4 (LT_PATH_NM): Try BSDization flags with a user-provided
+	NM, if there is one.  Run nm on itself, not on /dev/null, to avoid
+	errors from nms that refuse to work on non-regular files.  Remove
+	other workarounds for this problem.  Strip out blank lines from the
+	nm output.
+
 2021-09-27  Nick Alcock  <nick.alcock@oracle.com>
 
 	PR libctf/27967

libtool.m4

@@ -3200,53 +3200,55 @@ _LT_DECL([], [file_magic_cmd], [1],
 
 # LT_PATH_NM
 # ----------
-# find the pathname to a BSD- or MS-compatible name lister
+# find the pathname to a BSD- or MS-compatible name lister, and any flags
+# needed to make it compatible
 AC_DEFUN([LT_PATH_NM],
 [AC_REQUIRE([AC_PROG_CC])dnl
 AC_CACHE_CHECK([for BSD- or MS-compatible name lister (nm)], lt_cv_path_NM,
 [if test -n "$NM"; then
-  # Let the user override the test.
+   # Let the user override the nm to test.
-  lt_cv_path_NM="$NM"
+   lt_nm_to_check="$NM"
-else
+ else
-  lt_nm_to_check="${ac_tool_prefix}nm"
+   lt_nm_to_check="${ac_tool_prefix}nm"
-  if test -n "$ac_tool_prefix" && test "$build" = "$host"; then
+   if test -n "$ac_tool_prefix" && test "$build" = "$host"; then
-    lt_nm_to_check="$lt_nm_to_check nm"
+     lt_nm_to_check="$lt_nm_to_check nm"
-  fi
+   fi
-  for lt_tmp_nm in $lt_nm_to_check; do
+ fi
-    lt_save_ifs="$IFS"; IFS=$PATH_SEPARATOR
+ for lt_tmp_nm in $lt_nm_to_check; do
-    for ac_dir in $PATH /usr/ccs/bin/elf /usr/ccs/bin /usr/ucb /bin; do
+   lt_save_ifs="$IFS"; IFS=$PATH_SEPARATOR
-      IFS="$lt_save_ifs"
+   for ac_dir in $PATH /usr/ccs/bin/elf /usr/ccs/bin /usr/ucb /bin; do
-      test -z "$ac_dir" && ac_dir=.
+     IFS="$lt_save_ifs"
-      tmp_nm="$ac_dir/$lt_tmp_nm"
+     test -z "$ac_dir" && ac_dir=.
-      if test -f "$tmp_nm" || test -f "$tmp_nm$ac_exeext" ; then
+     case "$lt_tmp_nm" in
-	# Check to see if the nm accepts a BSD-compat flag.
+     */*|*\\*) tmp_nm="$lt_tmp_nm";;
-	# Adding the `sed 1q' prevents false positives on HP-UX, which says:
+     *) tmp_nm="$ac_dir/$lt_tmp_nm";;
-	#   nm: unknown option "B" ignored
+     esac
-	# Tru64's nm complains that /dev/null is an invalid object file
+     if test -f "$tmp_nm" || test -f "$tmp_nm$ac_exeext" ; then
-	case `"$tmp_nm" -B /dev/null 2>&1 | sed '1q'` in
+       # Check to see if the nm accepts a BSD-compat flag.
-	*/dev/null* | *'Invalid file or object type'*)
+       # Adding the `sed 1q' prevents false positives on HP-UX, which says:
-	  lt_cv_path_NM="$tmp_nm -B"
+       #   nm: unknown option "B" ignored
-	  break
+       case `"$tmp_nm" -B "$tmp_nm" 2>&1 | grep -v '^ *$' | sed '1q'` in
-	  ;;
+       *$tmp_nm*) lt_cv_path_NM="$tmp_nm -B"
-	*)
+	 break
-	  case `"$tmp_nm" -p /dev/null 2>&1 | sed '1q'` in
+	 ;;
-	  */dev/null*)
+       *)
-	    lt_cv_path_NM="$tmp_nm -p"
+	 case `"$tmp_nm" -p "$tmp_nm" 2>&1 | grep -v '^ *$' | sed '1q'` in
-	    break
+	 *$tmp_nm*)
-	    ;;
+	   lt_cv_path_NM="$tmp_nm -p"
-	  *)
+	   break
-	    lt_cv_path_NM=${lt_cv_path_NM="$tmp_nm"} # keep the first match, but
+	   ;;
-	    continue # so that we can try to find one that supports BSD flags
+	 *)
-	    ;;
+	   lt_cv_path_NM=${lt_cv_path_NM="$tmp_nm"} # keep the first match, but
-	  esac
+	   continue # so that we can try to find one that supports BSD flags
-	  ;;
+	   ;;
-	esac
+	 esac
-      fi
+	 ;;
-    done
+       esac
-    IFS="$lt_save_ifs"
+     fi
-  done
+   done
-  : ${lt_cv_path_NM=no}
+   IFS="$lt_save_ifs"
-fi])
+ done
+ : ${lt_cv_path_NM=no}])
 if test "$lt_cv_path_NM" != "no"; then
   NM="$lt_cv_path_NM"
 else