meson: don't pass "install" to configure_file() unless really needed

In our case it was never needed because it defaults to true if install_dir is set,
which it always is for all calls.

This avoids a warning when running with newer meson where it complains that install
is only available with 0.50+.

Fixes #298

meson.build

@@ -179,7 +179,6 @@ configure_file(input : 'gdump.c',
   output : 'gdump.c',
   copy: true,
   install_dir: join_paths(get_option('datadir'), 'gobject-introspection-1.0'),
-  install : true,
 )
 
 girepo_dep = declare_dependency(