Document get_variable(system)

docs/markdown/Dependencies.md

@@ -80,8 +80,8 @@ var = foo_dep.get_variable(cmake : 'CMAKE_VAR', pkgconfig : 'pkg-config-var', co
 ```
 
 It accepts the keywords 'cmake', 'pkgconfig', 'pkgconfig_define',
-'configtool', 'internal', and 'default_value'. 'pkgconfig_define'
-works just like the 'define_variable' argument to
+'configtool', 'internal', 'system', and 'default_value'.
+'pkgconfig_define' works just like the 'define_variable' argument to
 `get_pkgconfig_variable`. When this method is invoked the keyword
 corresponding to the underlying type of the dependency will be used to
 look for a variable. If that variable cannot be found or if the caller

docs/markdown/snippets/system_variable_in_dep.md

@@ -0,0 +1,3 @@
+## Support for variable in system dependencies
+
+System Dependency method `get_variable()` now supports `system` variable.

docs/yaml/objects/dep.yaml

@@ -191,7 +191,7 @@ methods:
         since: 0.58.0
         description: |
           This argument is used as a default value
-          for `cmake`, `pkgconfig`, `configtool` and `internal` keyword
+          for `cmake`, `pkgconfig`, `configtool`, `internal` and `system` keyword
           arguments. It is useful in the common case where `pkgconfig` and `internal`
           use the same variable name, in which case it's easier to write `dep.get_variable('foo')`
           instead of `dep.get_variable(pkgconfig: 'foo', internal: 'foo')`.
@@ -214,6 +214,11 @@ methods:
         since: 0.54.0
         description: The internal variable name
 
+      system:
+        type: str
+        since: 1.6.0
+        description: The system variable name
+
       default_value:
         type: str
         description: The default value to return when the variable does not exist