COMPILING: Pillow is a dependency, update Windows section

The Windows section predated Freedoom’s removal of symbolic links.
After that, it’s not so necessary anymore for all the software to be
built with Cygwin (as that allowed them to handle symbolic links
correctly), but our use of Make still effectively demands at least
minimal Unix-ish capabilities.

COMPILING.adoc

@@ -16,22 +16,18 @@ Building the Freedoom IWADs pretty much simply requires the following:
     https://www.gnu.org/software/make/[GNU Make] (patches to fix
     portability are most welcome!).  On non-GNU systems, it might be
     available in a package and binary named as 'gmake'.
-  * 'ImageMagick': This is required for generating some of the
-    graphics in Freedoom, including constructing text strings for
-    certain cases (such as the big font in level titles).  Some newer
-    versions of ImageMagick are known to produce
-    https://github.com/freedoom/freedoom/issues/41[strange results] on
-    certain operations, most notably the title screen graphic having
-    funny colors, but nothing major should be wrong.
   * 'Python': Freedoom uses several Python programs in its tree to
     assist with building the IWADs.  Both Python 2.7 and 3.x are fully
     supported.
+  * 'Pillow': A Python image manipulation module that provides the
+    features we need for scaling and composing various graphics of the
+    game.
   * 'DeuTex' 5.0: Freedoom depends on features developed in this
     version of DeuTex and will not build on earlier versions.  It is
-    available at https://github.com/Doom-Utils/deutex in source,
-    Windows binary, and Ubuntu binary formats.  As of July 2017, this
-    version is very recent and may not be widely available in other
-    distribution repositories yet.
+    available at https://github.com/Doom-Utils/deutex in source and
+    Windows binary formats.  As of July 2017, this version is very
+    recent and may not be widely available in Unix distribution
+    repositories yet.
 
 All or most of this software should already be available in your
 operating system's software repository, with the likely exception of
@@ -56,27 +52,19 @@ not GNU and it fails, try `gmake` instead.
 Building on Microsoft Windows
 -----------------------------
 
-Freedoom has normally seen all of its development on Unix-like
-systems, nevertheless some people like to compile Freedoom on Windows.
-This is possible, although complicated by our choice of tooling for
-the project.
+Freedoom has normally seen all of its development on Unix systems,
+nevertheless some people like to compile Freedoom on Windows.  This is
+possible, although complicated by our choice of tooling for the
+project.
 
-It may be possible to use regular Windows versions of all the required
-software, but https://cygwin.com/[Cygwin] is highly recommended to
-recreate the Unix environment on this operating system anyway.  It
-will simplify the process of building Freedoom greatly.  The
-previously-mentioned https://github.com/Doom-Utils/deutex[DeuTex
-repository] will require a C compiler (such as `gcc` or `clang`) to be
-installed in order to build.
+GNU environments for Windows such as https://cygwin.com/[Cygwin],
+http://www.msys2.org/[MSYS2], or
+https://blogs.msdn.microsoft.com/wsl/[WSL] provide sufficient shell
+capabilities to build Freedoom.  The dependencies listed previously
+for Freedoom apply just as well to a Windows environment.
 
-It is important that you install the Cygwin-specific versions of all
-the required software.  Native versions of Make, Python, ImageMagick,
-and DeuTex are not likely to interact well with Cygwin versions of the
-software.
-
-As Cygwin provides a GNU system and interface on top of Windows, its
-command line is (by default) Bash and not `cmd.exe`.  Instructions on
-how to use GNU systems and Bash are out of scope of this document.
+Instructions on how to use a GNU system and shell are out of scope of
+this document.
 
 Optional software
 -----------------