The source archive already has the 
cygwin-specific patch readline-4.2-X.patch applied.

To remove and revert to a 'clean' readline-4.2
  mv <srctop>/CYGWIN-PATCHES/readline-4.2-X.patch <srctop>/..
  cd <srctop>
  patch -p1 -R < ../readline-4.2-X.patch
  
------------------

Requirements:
  gcc-2.95.2-5 or later
       this was released in November 2000. The readline makefile uses
       new gcc switches and features introduced at that time) 
  binutils-20001029-2 or later 
       this was released in November 2000. The readline makefile uses
       new ld switches and features introduced in that binutils release)

Direct Dependencies:
  none

Canonical homepage:
http://www.gnu.org/software/readline/readline.html

Canonical download:
ftp://ftp.gnu.org/pub/gnu/readline/

------------------

To link this library with your projects:
  #1) include readline.h (and/or history.h, if you only want the history
      stuff)
  #2) to link with the static library, use '-static' on the link line and
      use '-DREADLINE_STATIC' when compiling your .c into a .o
  #3) to link with the dll compile and link as usual; no special flags
      or -Ddefines are necessary

To build the library for cygwin: 

First, insure that gcc finds the termcap.h file from the ncurses 
  distribution first, before the one from the termcap distribution.
  You can do this by adding 'CFLAGS=-I/usr/include/ncurses' to your
  configure line.
This is because readline uses termcap functions, but will link to 
  the ncurses library (which provides emulated versions of these
  termcap functions).  If you've got the wrong termcap.h header,
  the link will fail.

Also, make sure that configure can't find libtermcap.  It has builtin 
  preference for it, and will ignore '--with-curses' if it finds
  libtermcap.

mv /usr/lib/libtermcap.a /usr/lib/libtermcap.a.orig

unpack readline-4.2-X-src.tar.gz
cd readline-4.2

CFLAGS="-I/usr/include/ncurses" ./configure --prefix=/usr --with-curses
make
make shared
make examples
make test
make install # prefix=/tmp/usr
make install-shared # prefix=/tmp/usr
make install-examples # prefix=/tmp/usr

If you like, strip the dll's:
  strip /usr/bin/cygreadline5.dll
  strip /usr/bin/cyghistory5.dll

Don't forget to restore /usr/lib/libtermcap.a

------------------

Files included in the binary distro

LIBS:

  <prefix>/bin/cygreadline5.dll
  <prefix>/bin/cyghistory5.dll
  <prefix>/lib/libhistory.a
  <prefix>/lib/libhistory.dll.a
  <prefix>/lib/libreadline.a
  <prefix>/lib/libreadline.dll.a

INCLUDES:

  <prefix>/include/readline/chardefs.h
  <prefix>/include/readline/history.h
  <prefix>/include/readline/keymaps.h
  <prefix>/include/readline/readline.h
  <prefix>/include/readline/readline_dll.h
  <prefix>/include/readline/rlconf.h
  <prefix>/include/readline/rlstdc.h
  <prefix>/include/readline/tilde.h

PROGS:

  <prefix>/bin/fileman.exe
  <prefix>/bin/rl.exe
  <prefix>/bin/rltest.exe
  <prefix>/bin/rlversion.exe

  <prefix>/bin/fileman-stat.exe
  <prefix>/bin/rl-stat.exe
  <prefix>/bin/rltest-stat.exe
  <prefix>/bin/rlversion-stat.exe

DOCS:

  <prefix>/doc/CHANGELOG
  <prefix>/doc/CHANGES
  <prefix>/doc/COPYING
  <prefix>/doc/README
  <prefix>/doc/USAGE
  <prefix>/doc/Cygwin/readline-4.1.README
  <prefix>/info/history.info
  <prefix>/info/readline.info
  <prefix>/info/rluserman.info
  <prefix>/man/man3/readline.3

------------------

Port Notes:

As of readline-4.2, the API changed; some functions were changed to
  have the rl_ prefix.  If your program won't link, try editing your
  source code as follows:
    completion_matches           --> rl_completion_matches
    crlf                         --> rl_crlf
    ding                         --> rl_ding
    filename_completion_function --> rl_filename_completion_function
    free_undo_list               --> rl_free_undo_list
    maybe_replace_line           --> rl_maybe_replace_line
    maybe_save_line              --> rl_maybe_save_line
    maybe_unsave_line            --> rl_maybe_unsave_line
    username_completion_function --> rl_username_completion_function
  There were also a number of new functions added in readline-4.2, 
  but that doesn't affect backwards compatibility.

Since the function names for some functions were changed, the dll
from readline-4.2 is NOT backwards compatible with older executables.
Therefore, the DLL version number was bumped from the arbitrary "4"
to the equally arbitrary "5".

-- 4.1:

The dll's are named according to this pattern: "cygreadline4.dll"
  NOT "libreadline.dll".  This helps to avoid dll naming conflicts 
  between cygwin dll's and dll's from other platforms (native, pw,
  etc).  

The dll's are VERSIONED. (i.e. "cygreadline5.dll" NOT "cygreadline.dll")
  Again, this helps distinguish the dll's from later, possibly
  incompatible releases.  As long as newer releases are still
  API/ABI compatible, they will retain the same version number
  shown here (4). 

I had to export the xmalloc, xrealloc, and xfree functions from the 
  dll, even though they were originally marked as private to the 
  library.  Without exporting those functions, the 'fileman' program 
  won't build.  Hopefully this won't cause any namespace clashes
  with other programs...

Cygwin port maintained by: Charles Wilson cwilson@ece.gatech.edu