Remove support for the MinGW compiler packaged with Cygwin.

MinGW-w64 is still supported, as is the MinGW compiler distributed by
mingw.org.

Cygwin's MinGW toolchain uses the 4.0-1 mingw-runtime package by
default, which appears to correspond to the "WSL-4" 4.x version of the
MinGW project's mingwrt packages.  (The version numbers in the
/usr/i686-pc-mingw32/sys-root/mingw/include/_mingw.h file are also
consistent with this theory.)

The WSL-4 branch was repudiated by the MinGW project as a mistake [1].
The MinGW project claims no responsibility for Cygwin's MinGW packages
[2].  There were several releases in short succession [3]:
  4.0.3 2013-09-22
  4.0.2 2013-09-15
  4.0.1 2013-09-14
  4.0.0 2013-08-23
There have been no 4.x releases since then, though there have been more
3.x releases (3.21 and 3.21.1).  The FILE_FLAG_FIRST_PIPE_INSTANCE bug
that this project has worked around was fixed in 4.0.3 [4], which was
never propagated to the Cygwin package.

The MinGW packages were apparently orphaned on 2014-09-18 [5] and are
still orphaned [6].

The g++ compiler in this package is still 4.7.3 (released on 2013-04-11),
which is almost three three years old.  While writing the "GenRandom"
module, I hit a bug where a non-static data member initializer of
pointer-to-function-type segfaulted the compiler.  (gcc seemed to have
confused the field with a pure virtual method.)

[1] http://sourceforge.net/p/mingw/bugs/1673/#5260/41ec
[2] http://sourceforge.net/p/mingw/bugs/1673/#2556/35d4
[3] http://sourceforge.net/projects/mingw/files/MinGW/Base/mingwrt/
[4] http://sourceforge.net/p/mingw/bugs/2050/
[5] https://cygwin.com/ml/cygwin/2014-09/msg00289.html
[6] https://cygwin.com/cygwin-pkg-maint

README.rst

@@ -25,34 +25,35 @@ You need the following to build winpty:
 
 * A Cygwin or MSYS installation
 * GNU make
-* A MinGW g++ toolchain, v4 or later, to build ``winpty.dll`` and
-  ``winpty-agent.exe``
-* A g++ toolchain targeting Cygwin or MSYS, v3 or later, to build
-  ``console.exe``
+* A MinGW g++ toolchain capable of compiling C++11 code to build ``winpty.dll``
+  and ``winpty-agent.exe``
+* A g++ toolchain targeting Cygwin or MSYS to build ``console.exe``
 
 Winpty requires two g++ toolchains as it is split into two parts. The
-binaries winpty.dll and winpty-agent.exe interface with the native Windows
-command prompt window so they are compiled with the native MinGW toolchain.
-The console.exe binary interfaces with the MSYS/Cygwin terminal so it is
-compiled with the MSYS/Cygwin toolchain.
+``winpty.dll`` and ``winpty-agent.exe`` binaries interface with the native
+Windows command prompt window so they are compiled with the native MinGW
+toolchain.  The console.exe binary interfaces with the MSYS/Cygwin terminal so
+it is compiled with the MSYS/Cygwin toolchain.
 
 MinGW appears to be split into two distributions -- MinGW (creates 32-bit
 binaries) and MinGW-w64 (creates both 32-bit and 64-bit binaries).  Either
-one is acceptable, but the compiler must be v4 or later.
+one is generally acceptable.
 
 Cygwin packages
 ---------------
 
 The default g++ compiler for Cygwin targets Cygwin itself, but Cygwin also
-packages MinGW compilers from both the MinGW and MinGW-w64 projects.  As of
-this writing, the necessary packages are:
+packages MinGW-w64 compilers.  As of this writing, the necessary packages are:
 
-* Either ``mingw-gcc-g++``, ``mingw64-i686-gcc-g++`` or
-  ``mingw64-x86_64-gcc-g++``.  Select the appropriate compiler for your
-  CPU architecture.
+* Either ``mingw64-i686-gcc-g++`` or ``mingw64-x86_64-gcc-g++``.  Select the
+  appropriate compiler for your CPU architecture.
 * ``gcc-g++``
 * ``make``
 
+As of this writing (2016-01-23), only the MinGW-w64 compiler is acceptable.
+The MinGW compiler (e.g. from the ``mingw-gcc-g++`` package) is no longer
+maintained and is too buggy.
+
 MSYS packages
 -------------
 

configure

@@ -64,7 +64,7 @@ case $(uname -s) in
             i686)
                 echo 'uname -m identifies an i686 environment.'
                 UNIX_CXX=i686-pc-cygwin-g++
-                MINGW_CXX="i686-w64-mingw32-g++ i686-pc-mingw32-g++"
+                MINGW_CXX=i686-w64-mingw32-g++
                 ;;
             x86_64)
                 echo 'uname -m identifies an x86_64 environment.'

src/libwinpty/winpty.cc

@@ -33,12 +33,6 @@
 #include "../shared/AgentMsg.h"
 #include "../shared/Buffer.h"
 
-// Work around a bug with mingw-gcc-g++.  mingw-w64 is unaffected.  See
-// GitHub issue 27.
-#ifndef FILE_FLAG_FIRST_PIPE_INSTANCE
-#define FILE_FLAG_FIRST_PIPE_INSTANCE 0x00080000
-#endif
-
 // TODO: Error handling, handle out-of-memory.
 
 #define AGENT_EXE L"winpty-agent.exe"