docs: add documentation for config_tool variable method

docs/markdown/Reference-manual.md

@@ -1608,6 +1608,10 @@ an external dependency with the following methods:
    pkg-config variable specified, or, if invoked on a non pkg-config
    dependency, error out
 
+ - `get_configtool_variable(varname)` (*Added 0.44.0*) will get the 
+   command line argument from the config tool (with `--` prepended), or,
+   if invoked on a non config-tool dependency, error out.
+
  - `type_name()` which returns a string describing the type of the
    dependency, the most common values are `internal` for deps created
    with `declare_dependencies` and `pkgconfig` for system dependencies

docs/markdown/snippets/config-tool-variable-method.md

@@ -0,0 +1,11 @@
+# Config-Tool based dependencies gained a method to get arbitrary options
+
+A number of dependencies (CUPS, LLVM, pcap, WxWidgets, GnuStep) use a config
+tool instead of pkg-config. As of this version they now have a
+`get_configtool_variable` method, which is analogous to the
+`get_pkgconfig_variable` for pkg config.
+
+```meson
+dep_llvm = dependency('LLVM')
+llvm_inc_dir = dep_llvm.get_configtool_variable('includedir')
+```