build(meson): use override_dependency if supported (#116)

This improves user experience when using tomlplusplus as a Meson
subproject.

With this change users will be able to simply call
dependency('tomlplusplus') and have Meson automatically resolve the
dependency from the subproject if it is not found on the system.

Without this, users always have to explicitly call
dependency('tomlplusplus', fallback:['tomlplusplus','tomlplusplus_dep'])

A lot of saved keystrokes :)

meson.build

@@ -508,6 +508,10 @@ tomlplusplus_dep = declare_dependency(
 	version: meson.project_version(),
 )
 
+if meson.version().version_compare('>=0.54.0')
+	meson.override_dependency('tomlplusplus', tomlplusplus_dep)
+endif
+
 if not is_subproject
 	import('pkgconfig').generate(
 		name: meson.project_name(),