Improve libcurl detection.

* m4/libcurl.m4: Do not use AC_PATH_PROG if --with-libcurl as been
given.  Suggested by John Marshall.
--

GnuPG-bug-id: 1510

m4/libcurl.m4

@@ -75,7 +75,11 @@ AC_DEFUN([LIBCURL_CHECK_CONFIG],
      if test -d "$_libcurl_with" ; then
         LIBCURL_CPPFLAGS="-I$withval/include"
         _libcurl_ldflags="-L$withval/lib"
-        AC_PATH_PROG([_libcurl_config],["$withval/bin/curl-config"])
+        if test -x "$withval/bin/curl-config" ; then
+          _libcurl_config="$withval/bin/curl-config"
+        else
+          _libcurl_config=
+        fi
      else
 	AC_PATH_PROG([_libcurl_config],[curl-config])
      fi