bug#42411: Bug with M-x compile

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

Thanks for fixing (the second part of) bug#42383.

The first part remains unfixed, however:

There are too many completion candidates in the list of targets when 
completing M-x compile.  For example, for the Makefile 
"foo:\n\techo\x20bar:\n" three candidates are displayed: "foo", "echo" and 
"bar".  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-*".  If one really wants to allow spaces (but 
not TABs) at the beginning of a target label, the following regexp could 
also be used: "^ *\\([^\t\n#%.$][^:=\n]*\\)\\s-*:[^=]".

The current regexp is an old one (since Emacs 21 at least), and is 
inconsistent with for example how bash computes completions (see 
_make_target_extract_script).

If changing the regexp is not an option, please make it a configuration 
option.

Gregory