[libxml2] Add instructions on how to use CMake to compile libxml

[Nick Wellnhofer <nwellnhof src gnome org>]

commit 4fc473d7e884ce5315c85febf28c2daeaca9ae0b
Author: Markus Rickert <rickert fortiss org>
Date:   Sun May 16 17:48:07 2021 +0200

    Add instructions on how to use CMake to compile libxml

 INSTALL.libxml2 | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)
---
diff --git a/INSTALL.libxml2 b/INSTALL.libxml2
index ac9211d6..d369ef69 100644
--- a/INSTALL.libxml2
+++ b/INSTALL.libxml2
@@ -27,6 +27,26 @@ Compilation
 
      Please report test failures to the mailing list or bug tracker.
 
+     Another option for compiling libxml is using CMake:
+
+         cmake -E tar xf libxml2-xxx.tar.gz
+         cmake -S libxml2-xxx -B libxml2-xxx-build [possible options]
+         cmake --build libxml2-xxx-build
+         cmake --install libxml2-xxx-build
+
+     Common CMake options include:
+
+         -D BUILD_SHARED_LIBS=OFF            # build static libraries
+         -D CMAKE_BUILD_TYPE=Release         # specify build type
+         -D CMAKE_INSTALL_PREFIX=/usr/local  # specify the install path
+         -D LIBXML2_WITH_ICONV=OFF           # disable iconv
+         -D LIBXML2_WITH_LZMA=OFF            # disable liblzma
+         -D LIBXML2_WITH_PYTHON=OFF          # disable Python
+         -D LIBXML2_WITH_ZLIB=OFF            # disable libz
+
+     You can also open the libxml source directory with its CMakeLists.txt
+     directly in various IDEs such as CLion, QtCreator, or Visual Studio.
+
   2. What other libraries are needed to compile/install libxml?
 
      Libxml does not require any other libraries. A platform with somewhat