Use dependency() with subproject fallback for deps

None of these libraries are packaged anywhere right now since they do
not have Meson build definitions, but in case they ever do it should be
possible to use them from a system package manager.

CHANGELOG.md

@@ -10,6 +10,9 @@ Versioning](https://semver.org/spec/v2.0.0.html).
 
 ### Packaging
 
+- The Meson wrap dependencies for `bitsery`, `function2` and `tomlplusplus` are
+  now defined using `dependency()` with a subproject fallback to ease distro
+  packaging.
 - The VST3 SDK Meson wrap now the `v3.7.1_build_50-patched` tag instead of a
   commit hash.
 

meson.build

@@ -278,10 +278,12 @@ boost_filesystem_dep = dependency(
   modules : ['filesystem'],
   static : with_static_boost,
 )
-bitsery_dep = subproject('bitsery', version : '5.2.0').get_variable('bitsery_dep')
-function2_dep = subproject('function2', version : '4.1.0').get_variable('function2_dep')
+# TODO: Meson doesn't have a way to define version ranges, does it? Like
+#       `^>=5.2.0`, `>=5.2.0 && <6.0.0` or `5.2.*`.
+bitsery_dep = dependency('bitsery', version : '>=5.2.0', fallback : ['bitsery', 'bitsery_dep'])
+function2_dep = dependency('function2', version : '>=4.1.0', fallback : ['function2', 'function2_dep'])
 threads_dep = dependency('threads')
-tomlplusplus_dep = subproject('tomlplusplus', version : '2.1.0').get_variable('tomlplusplus_dep')
+tomlplusplus_dep = dependency('tomlplusplus', version : '>=2.1.0', fallback : ['tomlplusplus', 'tomlplusplus_dep'])
 # The built in threads dependency does not know how to handle winegcc
 wine_threads_dep = declare_dependency(link_args : '-lpthread')
 xcb_dep = dependency('xcb')