Add meson build system

Meson is a build system somewhat like cmake, but without all of the
rough edges. It supports many OSes, including all of the major ones, and
a large number of C++ compilers.

My interest isn't really in convincing people to use meson as the
default here, but meson provides a subproject mechanism that can fetch
external projects and build them along with the main project in a single
configure/compile invocation. This is extremely useful for platforms
that lack a (competent) package manager.

As far as I know the meson build does everything the cmake build does,
with one exception: generate the cmake config/version files. meson can
generate these files, but only in simple cases, and not when using
export targets like tinyxml2 does.

readme.md

@@ -265,9 +265,31 @@ And additionally a test file:
 * xmltest.cpp
 
 Simply compile and run. There is a visual studio 2019 project included, a simple Makefile,
-an Xcode project, a Code::Blocks project, and a cmake CMakeLists.txt included to help you.
-The top of tinyxml.h even has a simple g++ command line if you are using Unix/Linux/BSD and
-don't want to use a build system.
+an Xcode project, a Code::Blocks project, a cmake CMakeLists.txt, and a meson.build are
+included to help you. The top of tinyxml.h even has a simple g++ command line if you are
+using Unix/Linux/BSD and don't want to use a build system.
+
+Using as a Meson Subproject
+---------------------------
+
+Create a wrap file such as:
+```ini
+[wrap-git]
+url = https://github.com/leethomason/tinyxml2.git
+revision = 8.0.1  # this can be any commit-ish (tag, sha) or the special value `head`
+```
+
+or, if you prefer to not use git
+
+```ini
+[wrap-file]
+directory = tinyxml2-8.0.1  # this is the name of the directory after de-compressing
+source_url = https://github.com/leethomason/tinyxml2/archive/8.0.1.tar.gz
+source_hash = sha256sum of compressed sources
+```
+
+in your project's `subprojects/` folder, and follow the meson documentation
+for using fallbacks.
 
 Building TinyXML-2 - Using vcpkg
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