8280863: Update build README to reflect that MSYS2 is supported

Reviewed-by: clanger
Backport-of: 204e67a

doc/building.html

@@ -195,7 +195,7 @@ <h2 id="operating-system-requirements">Operating System Requirements</h2>
 <h3 id="windows">Windows</h3>
 <p>Windows XP is not a supported platform, but all newer Windows should be able to build the JDK.</p>
 <p>On Windows, it is important that you pay attention to the instructions in the <a href="#special-considerations">Special Considerations</a>.</p>
-<p>Windows is the only non-POSIX OS supported by the JDK, and as such, requires some extra care. A POSIX support layer is required to build on Windows. Currently, the only supported such layer is Cygwin. (Msys is no longer supported due to a too old bash; msys2 and the new Windows Subsystem for Linux (WSL) would likely be possible to support in a future version but that would require effort to implement.)</p>
+<p>Windows is the only non-POSIX OS supported by the JDK, and as such, requires some extra care. A POSIX support layer is required to build on Windows. Currently, the only supported such layers are Cygwin and MSYS2. (MSYS is no longer supported due to an outdated bash; While OpenJDK can be built with MSYS2, support for it is still experimental, so build failures and unusual errors are not uncommon.)</p>
 <p>Internally in the build system, all paths are represented as Unix-style paths, e.g. <code>/cygdrive/c/git/jdk/Makefile</code> rather than <code>C:\git\jdk\Makefile</code>. This rule also applies to input to the build system, e.g. in arguments to <code>configure</code>. So, use <code>--with-msvcr-dll=/cygdrive/c/msvcr100.dll</code> rather than <code>--with-msvcr-dll=c:\msvcr100.dll</code>. For details on this conversion, see the section on <a href="#fixpath">Fixpath</a>.</p>
 <h4 id="cygwin">Cygwin</h4>
 <p>A functioning <a href="http://www.cygwin.com/">Cygwin</a> environment is thus required for building the JDK on Windows. If you have a 64-bit OS, we strongly recommend using the 64-bit version of Cygwin.</p>

doc/building.md

@@ -177,10 +177,9 @@ On Windows, it is important that you pay attention to the instructions in the
 
 Windows is the only non-POSIX OS supported by the JDK, and as such, requires
 some extra care. A POSIX support layer is required to build on Windows.
-Currently, the only supported such layer is Cygwin. (Msys is no longer
-supported due to a too old bash; msys2 and the new Windows Subsystem for Linux
-(WSL) would likely be possible to support in a future version but that would
-require effort to implement.)
+Currently, the only supported such layers are Cygwin and MSYS2. (MSYS is no longer
+supported due to an outdated bash; While OpenJDK can be built with MSYS2,
+support for it is still experimental, so build failures and unusual errors are not uncommon.)
 
 Internally in the build system, all paths are represented as Unix-style paths,
 e.g. `/cygdrive/c/git/jdk/Makefile` rather than `C:\git\jdk\Makefile`. This