add meson.get_native_property for native files

This allows Meson native-file [properties] to be used.
This avoids the need to call meson from a script file or have a
long command line invocation of `meson setup`

The method meson.get_native_property('prop', 'fallback') is added.
The native file can contain properties like

```
[properties]
myprop1 = 'foo'
mydir2 = 'lib/custom'
```

Then from within `meson.build`

```meson
x1 = meson.get_native_property('myprop1')

thedir = meson.get_native_property('mydir2', 'libs')
```

fallback values are optional

docs/markdown/snippets/native_property.md

@@ -0,0 +1,16 @@
+## Native file properties
+
+As of Meson 0.53.0, the `--native-file foo.txt` can contain:
+
+* binaries
+* paths
+* properties
+
+which are defined and used the same way as in cross files.
+The `properties` are new for Meson 0.53.0, and are read like:
+
+```meson
+x = meson.get_native_property('foobar', 'foo')
+```
+
+where `foobar` is the property name, and the optional `foo` is the fallback string value.

mesonbuild/environment.py

@@ -538,6 +538,7 @@ class Environment:
                 coredata.load_configs(self.coredata.config_files))
             binaries.build = BinaryTable(config.get('binaries', {}))
             paths.build = Directories(**config.get('paths', {}))
+            properties.build = Properties(config.get('properties', {}))
 
         ## Read in cross file(s) to override host machine configuration
 

mesonbuild/interpreter.py

@@ -1845,6 +1845,7 @@ class MesonMain(InterpreterObject):
                              'version': self.version_method,
                              'project_name': self.project_name_method,
                              'get_cross_property': self.get_cross_property_method,
+                             'get_native_property': self.get_native_property_method,
                              'backend': self.backend_method,
                              })
 
@@ -2031,6 +2032,22 @@ class MesonMain(InterpreterObject):
                 return args[1]
             raise InterpreterException('Unknown cross property: %s.' % propname)
 
+    @noArgsFlattening
+    @permittedKwargs({})
+    def get_native_property_method(self, args, kwargs):
+        if len(args) < 1 or len(args) > 2:
+            raise InterpreterException('Must have one or two arguments.')
+        propname = args[0]
+        if not isinstance(propname, str):
+            raise InterpreterException('Property name must be string.')
+        try:
+            props = self.interpreter.environment.properties.build
+            return props[propname]
+        except Exception:
+            if len(args) == 2:
+                return args[1]
+            raise InterpreterException('Unknown native property: %s.' % propname)
+
 
 known_library_kwargs = (
     build.known_shlib_kwargs |

mesonbuild/mesonlib.py

@@ -1,4 +1,4 @@
-# Copyright 2012-2015 The Meson development team
+# Copyright 2012-2019 The Meson development team
 
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.