Generate native gems with -fvisibility=hidden

I recently ran into very nasty issues with dynamic symbols clashing between
two native gems.

I believe the overwhelming majority of native gems don't want to export
their symbols, so hidding them by default would make sense to me.

bundler/lib/bundler/templates/newgem/ext/newgem/extconf-c.rb.tt

@@ -2,4 +2,6 @@
 
 require "mkmf"
 
+$CFLAGS << " -fvisibility=hidden "
+
 create_makefile(<%= config[:makefile_path].inspect %>)

bundler/lib/bundler/templates/newgem/ext/newgem/newgem.c.tt

@@ -2,7 +2,7 @@
 
 VALUE rb_m<%= config[:constant_array].join %>;
 
-void
+RUBY_FUNC_EXPORTED void
 Init_<%= config[:underscored_name] %>(void)
 {
   rb_m<%= config[:constant_array].join %> = rb_define_module(<%= config[:constant_name].inspect %>);

bundler/lib/bundler/templates/newgem/rubocop.yml.tt

@@ -9,5 +9,9 @@ Style/StringLiteralsInInterpolation:
   Enabled: true
   EnforcedStyle: double_quotes
 
+Style/GlobalVars:
+  Exclude:
+    - ext/**/extconf.rb
+
 Layout/LineLength:
   Max: 120