Remove references to cygwin in favour of Windows instead

Because cygwin is not tested in CI and because this means that we can
list operating systems rather than a mix of operating systems and tool
chains.

Author: thomasspriggs

COMPILING.md

@@ -5,7 +5,7 @@ following environments:
 
 - Linux
 - MacOS X
-- Microsoft Visual Studio 2019
+- Windows
 
 The above environments are currently tested as part of our continuous
 integration system. It separately tests both the CMake build system and the
@@ -17,7 +17,6 @@ past, but are not actively tested:
 
 - Solaris 11
 - FreeBSD 11
-- Cygwin
 
 # Building using CMake
 
@@ -203,66 +202,6 @@ Follow these instructions:
    make -C jbmc/src
    ```
 
-## COMPILATION ON WINDOWS WITH CYGWIN
-
-There are two options: the Visual Studio compiler with version 14 (2015) or
-later, or the MinGW cross compiler with version 5.4 or later.
-We recommend Visual Studio.
-
-Follow these instructions:
-
-1. First install Cygwin, then from the Cygwin setup facility install the
-   following packages: `flex, bison, tar, gzip, git, make, wget, patch,
-   curl`.
-2. Get the CBMC source via
-   ```
-   git clone https://github.com/diffblue/cbmc cbmc-git
-   cd cbmc-git
-   ```
-3. Depending on your choice of compiler:
-   1. To compile with Visual Studio, change the second line of `src/config.inc`
-      to
-      ```
-      BUILD_ENV = MSVC
-      ```
-      Open the Developer Command Prompt for Visual Studio, then start the
-      Cygwin shell with
-      ```
-      bash.exe -login
-      ```
-      Please note that this might open a different shell instead, especially if
-      you have installed other Linux subsystems previously. To verify that you
-      are in the correct shell, make sure that the Windows file system can be
-      accessed via the folder`/cygdrive`. If the command above does not open
-      the Cygwin shell, you can also access it by using its absolute path,
-      `C:\cygwin64\bin\bash.exe` by default. In the Developer Command Prompt, 
-      simply type
-      ```
-      C:\cygwin64\bin\bash.exe -login
-      ```
-   2. To compile with MinGW, use Cygwin setup to install a mingw g++ compiler
-      package, i.e. one of `mingw{32,64}-{x86_64,i686}-gcc-g++`. You may also
-      have to adjust the section in `src/common` that defines `CC` and `CXX`
-      for BUILD_ENV = Cygwin.
-      Then start the Cygwin shell.
-4. To compile CMBC, open the Cygwin shell and type
-   ```
-   make -C src DOWNLOADER=wget minisat2-download
-   make -C src
-   ```
-5. To compile JBMC, you additionally need the JDK and Maven 3, which have
-   to be installed manually. Then open the Cygwin shell and type
-   ```
-   make -C jbmc/src setup-submodules
-   make -C jbmc/src
-   ```
-
-(Optional) A Visual Studio project file can be generated with the script
-"generate_vcxproj" that is in the subdirectory "scripts".  The project file is
-helpful for GUI-based tasks, e.g., the class viewer, debugging, etc., and can
-be used for building with MSBuild.  Note that you still need to run flex/bison
-using "make generated_files" before opening the project.
-
 ## COMPILATION ON SOLARIS 11
 
 We assume Solaris 11.4 or newer.  To build JBMC, you'll need to install