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.build

@@ -0,0 +1,104 @@
+# 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.
+
+project(
+    'tinyxml2',
+    ['cpp'],
+    version : '8.0.0',
+    meson_version : '>= 0.49.0',
+)
+
+cpp = meson.get_compiler('cpp')
+
+tinyxml_extra_args = []
+if cpp.get_argument_syntax() == 'msvc'
+    tinyxml_extra_args += '-D_CRT_SECURE_NO_WARNINGS'
+endif
+
+if get_option('default_library') == 'shared'
+    tinyxml_extra_args += '-DTINYXML2_EXPORT'
+endif
+
+if get_option('debug')
+    tinyxml_extra_args += '-DTINYXML2_DEBUG'
+endif
+
+lib_tinyxml2 = library(
+    'tinyxml2',
+    ['tinyxml2.cpp'],
+    cpp_args : tinyxml_extra_args,
+    gnu_symbol_visibility : 'hidden',
+    version : meson.project_version(),
+    install : true,
+)
+
+dep_tinxyml2 = declare_dependency(
+    link_with : lib_tinyxml2,
+    include_directories : include_directories('.'),
+)
+
+# This is the new way to set dependencies, but let's not break users of older
+# versions of meson
+if meson.version().version_compare('>= 0.54.0')
+  meson.override_dependency('tinyxml2', dep_tinxyml2)
+endif
+
+if get_option('tests')
+    # Try to find a copy command. If this is windows we probably don't have cp,
+    # but if this is msys then we do, so make cp not required in that case, and
+    # try Xcopy if cp isn't found
+    prog_cp = find_program('cp', required : build_machine.system() != 'windows')
+    command = ['-r']
+    if not prog_cp.found()
+        prog_cp = find_program('Xcopy')
+        command = ['/E', '/I']
+    endif
+
+    # Copy the test resources into the build dir
+    run_command(
+        prog_cp,
+        [
+            command,
+            meson.current_source_dir() / 'resources',
+            meson.current_build_dir(),
+        ],
+    )
+
+    test(
+        'xmltest',
+        executable(
+            'xmltest',
+            ['xmltest.cpp'],
+            link_with : [lib_tinyxml2],
+        ),
+        workdir : meson.current_build_dir(),
+    )
+endif
+
+install_headers('tinyxml2.h')
+
+# This is better than using the .in because meson tracks dependencies
+# internally, and will generate a more accurate pkg-config file
+pkg = import('pkgconfig')
+pkg.generate(
+    lib_tinyxml2,
+    description : 'simple, small, C++ XML parser',
+)