bug#42383: 28.0.50; Two bugs with M-x compile

[Gregory Heytings via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

** Bug 1 **

Steps to reproduce:

1. create a Makefile with a few targets
2. start emacs -Q
3. M-x compile
4. press TAB: the list of targets is displayed
5. type the first character of an existing target, and press TAB
6. the result is a "[No match]"

This has been working correctly for years (tested with Emacs 24, 25, 26), 
and does not work anymore with Emacs 28.

It seems that at step 5 above the list of completion candidates that is 
considered are subdirectories.

** Bug 2 **

There are too many completion candidates for the list of targets.  For 
example, for the Makefile "foo:\n\techo foo:\n" two candidates are 
displayed: "foo" and "echo".  It seems that the regexp in 
pcmpl-gnu-make-targets is too large, and should be fixed as follows:

--- pcmpl-gnu.el.orig	2020-06-29 17:39:26.000000000 +0000
+++ pcmpl-gnu.el	2020-07-15 22:43:14.368938346 +0000
@@ -118,7 +118,7 @@
  Return the new list."
    (goto-char (point-min))
    (while (re-search-forward
-	  "^\\s-*\\([^\n#%.$][^:=\n]*\\)\\s-*:[^=]" nil t)
+          "^\\([^\t\n#%.$][^:=\n]*\\)\\s-*:[^=]" nil t)
      (setq targets (nconc (split-string (match-string-no-properties 1))
                           targets)))
    targets)

I see no reason to allow one or more TABs or spaces at the beginning of 
targets, as does the "^\\s-*".

Gregory