*/plugins/meson.build: define feature macros in Features.h

This makes ccache more efficient when recompiling with different
plugins.

src/input/meson.build

@@ -1,3 +1,5 @@
+input_features = configuration_data()
+
 input_api = static_library(
   'input_api',
   'Error.cxx',
@@ -54,3 +56,5 @@ input_glue_dep = declare_dependency(
     tag_dep,
   ],
 )
+
+configure_file(output: 'Features.h', configuration: input_features)