Add `main` argument to `dependency` (#2828)

`GTest` uses this keyword for conditional linking of it's main().

Fixes issue #2828

mesonbuild/interpreter.py

@@ -1368,7 +1368,7 @@ permitted_kwargs = {'add_global_arguments': {'language'},
                     'build_target': build_target_kwargs,
                     'configure_file': {'input', 'output', 'configuration', 'command', 'install_dir', 'capture', 'install'},
                     'custom_target': {'input', 'output', 'command', 'install', 'install_dir', 'build_always', 'capture', 'depends', 'depend_files', 'depfile', 'build_by_default'},
-                    'dependency': {'default_options', 'fallback', 'language', 'method', 'modules', 'optional_modules', 'native', 'required', 'static', 'version'},
+                    'dependency': {'default_options', 'fallback', 'language', 'main', 'method', 'modules', 'optional_modules', 'native', 'required', 'static', 'version'},
                     'declare_dependency': {'include_directories', 'link_with', 'sources', 'dependencies', 'compile_args', 'link_args', 'version'},
                     'executable': exe_kwargs,
                     'find_program': {'required', 'native'},