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.

meson_options.txt

@@ -0,0 +1,28 @@
+# Copyright © 2020 Dylan Baker
+
+# This software is provided 'as-is', without any express or implied
+# warranty. In no event will the authors be held liable for any
+# damages arising from the use of this software.
+
+# Permission is granted to anyone to use this software for any
+# purpose, including commercial applications, and to alter it and
+# redistribute it freely, subject to the following restrictions:
+
+# 1. The origin of this software must not be misrepresented; you must
+# not claim that you wrote the original software. If you use this
+# software in a product, an acknowledgment in the product documentation
+# would be appreciated but is not required.
+
+# 2. Altered source versions must be plainly marked as such, and
+# must not be misrepresented as being the original software.
+
+# 3. This notice may not be removed or altered from any source
+# distribution.
+
+option(
+    'tests',
+    type : 'boolean',
+    description : 'Enable unit tests',
+    value : true,
+    yield : true,
+)