More Meson work (#1025)

This is follow up work to my previous series. I've tried to make the
Meson build mirror the CMake build more closely. I've also made an
attempt at adding some instructions to the documents on using Meson.

---------

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Henry Schreiner <henryschreineriii@gmail.com>

book/chapters/installation.md

@@ -51,7 +51,7 @@ If the CMake option `CLI11_PRECOMPILED` is set then the library is compiled into
 a static library. This can be used to improve compile times if CLI11 is included
 in many different parts of a project.
 
-### Global Headers
+#### Global Headers
 
 Use `CLI/*.hpp` files stored in a shared folder. You could check out the git
 repository to a system-wide folder, for example `/opt/`. With CMake, you could
@@ -194,6 +194,32 @@ default to off if CLI11 is used as a subdirectory in another project.
     nothing special about this container. Alpine is being used because it is
     small, modern, and fast. Commands are similar on any other platform.
 
+## Meson support
+
+### Global Headers from pkg-config
+
+If CLI11 is installed globally, then nothing more than `dependency('CLI11')` is
+required. If it installed in a non-default search path, then setting the
+`PKG_CONFIG_PATH` environment variable of the `--pkg-config-path` option to
+`meson setup` is all that's required.
+
+### Using Meson's subprojects
+
+Meson has a system called
+[wraps](https://mesonbuild.com/Wrap-dependency-system-manual.html), which allow
+Meson to fetch sources, configure, and build dependencies as part of a main
+project. This is the mechanism that Meson recommends for projects to use, as it
+allows updating the dependency transparently, and allows packagers to have fine
+grained control on the use of subprojects vs system provided dependencies.
+Simply run `meson wrap install cli11` to install the `cli11.wrap` file, and
+commit it, if desired.
+
+It is also possible to use git submodules. This is generally discouraged by
+Meson upstream, but may be appropriate if a project needs to build with multiple
+build systems and wishes to share subprojects between them. As long as the
+submodule is in the parent project's subproject directory nothing additional is
+needed.
+
 ## Installing cli11 using vcpkg
 
 You can download and install cli11 using the