Fix build failure; ref #5194

Fix the following two issues:

1. Compile failure when external gem is specified.
   * https://github.com/mruby/mruby/pull/5194#issuecomment-734303442
2. When there are multiple build targets, presym parsing sometimes fails or
   output result of presym is broken.
   * https://github.com/mruby/mruby/pull/5194#issuecomment-734322142

The root cause of 2 is unknown, but it seems to occur when presym parsing is
performed in parallel, therefore I change them so that they are not run in
parallel.

1 files changed, 3 insertions, 1 deletions

diff --git a/Rakefile b/Rakefile
index c28486b01..8be192155 100644
--- a/Rakefile
+++ b/Rakefile
@@ -93,7 +93,9 @@ MRuby.each_target do |target|
 end
 
 desc "preallocated symbols"
-task :gensym => MRuby.targets.values.map(&:presym_file)
+task :gensym do
+  MRuby.each_target{|build| Rake::Task[build.presym_file].invoke}
+end
 
 depfiles += MRuby.targets.map { |n, t|
   t.libraries