acinclude: remove PKGCONFIG override

... and allow it to get set by a caller easier.

Reported-by: Rainer Jung
Bug: http://curl.haxx.se/mail/lib-2015-10/0035.html

acinclude.m4

@@ -5,7 +5,7 @@
 #                            | (__| |_| |  _ <| |___
 #                             \___|\___/|_| \_\_____|
 #
-# Copyright (C) 1998 - 2013, Daniel Stenberg, <daniel@haxx.se>, et al.
+# Copyright (C) 1998 - 2015, Daniel Stenberg, <daniel@haxx.se>, et al.
 #
 # This software is licensed as described in the file COPYING, which
 # you should have received as part of this distribution. The terms
@@ -3052,9 +3052,6 @@ dnl Optionally PKG_CONFIG_LIBDIR may be given as $pcdir.
 dnl
 
 AC_DEFUN([CURL_CHECK_PKGCONFIG], [
-
-    PKGCONFIG="no"
-
     AC_PATH_TOOL( PKGCONFIG, pkg-config, no, $PATH:/usr/bin:/usr/local/bin)
 
     if test x$PKGCONFIG != xno; then