configure: set PATH_SEPARATOR to colon for PATH w/o separator

The logic tries to figure out what the path separator in the $PATH
variable is, but if there's only one directory in the $PATH it
fails. This change make configure *guess* on colon instead of erroring
out, simply because that is probably the more common character.

PATH_SEPARATOR can always be set by the user to override the guessing.

(tricky bug to reproduce, as in my case for example the configure script
requires binaries in more than one directory so passing in a PATH with a
single dir fails.)

Reported-by: Earnestly on github
Fixes #2202
Closes #2265

m4/zz40-xc-ovr.m4

@@ -2,7 +2,7 @@
 #
 # zz40-xc-ovr.m4
 #
-# Copyright (c) 2013 Daniel Stenberg <daniel@haxx.se>
+# Copyright (c) 2013 - 2018 Daniel Stenberg <daniel@haxx.se>
 #
 # Permission to use, copy, modify, and distribute this software for any
 # purpose with or without fee is hereby granted, provided that the above
@@ -18,9 +18,6 @@
 #
 #---------------------------------------------------------------------------
 
-# serial 1
-
-
 dnl The funny name of this file is intentional in order to make it
 dnl sort alphabetically after any libtool, autoconf or automake
 dnl provided .m4 macro file that might get copied into this same
@@ -398,9 +395,10 @@ if test $xc_tst_dirs_sem -eq $xc_tst_dirs_col; then
   # When both counting methods give the same result we do not want to
   # chose one over the other, and consider auto-detection not possible.
   if test -z "$PATH_SEPARATOR"; then
-    # Stop dead until user provides 'PATH_SEPARATOR' definition.
-    echo "$xc_msg_err 'PATH_SEPARATOR' variable not set. $xc_msg_abrt" >&2
-    exit 1
+    # User should provide the correct 'PATH_SEPARATOR' definition.
+    # Until then, guess that it is colon!
+    echo "$xc_msg_warn path separator not determined, guessing colon" >&2
+    PATH_SEPARATOR=':'
   fi
 else
   # Separator with the greater directory count is the auto-detected one.