--with-readline=DIR doesn't work with CVS HEAD

[Jun T. <tak...@kb...>]

"--with-readline=/usr/local" does not work on Mac OS X after the
following patch (configure.ac, revision 1.21):
----------------------------
revision 1.21
date: 2016/05/23 19:02:18;  author: markisch;  state: Exp;  lines: +34 -36
Detect if --with-readline=DIR refers to GNU readline or NetBSD editline.
----------------------------

On Mac OS X, Apple installs editline (libedit) in /usr/{include,lib}
but I want to use GNU readline instead. So I've installed GNU readline
in /usr/local/{include,lib}, and was configuring gnuplot as:

./configure --with-readline=/usr/local ...

This was working fine, but after the above patch configure started to use
/usr/lib/libedit.dylib even if "--with-readline=/usr/local" is specified.
I want to use /usr/local/lib/libreadline.dylib instead.

The problem is at line 447 of configure.ac:

      AC_CHECK_HEADERS(editline/readline.h,[with_readline=bsd],,)               

This finds /usr/include/editline/readline.h and set with_readline=bsd
even if -I/usr/local/include is added to CPPFLAGS (configure.ac:441).

A possible patch is attached.

Jun