Do not overwrite `conf.cc.defines`.
MODIFY
appveyor_config.rb
appveyor_config.rb
@@ -17,7 +17,7 @@ MRuby::Build.new('full-debug') do |conf|
# include all core GEMs
conf.gembox 'full-core'
- conf.cc.defines = %w(MRB_ENABLE_DEBUG_HOOK)
+ conf.cc.defines += %w(MRB_ENABLE_DEBUG_HOOK)
conf.enable_test
end

MODIFY
travis_config.rb
travis_config.rb
@@ -18,7 +18,7 @@ MRuby::Build.new('full-debug') do |conf|
# include all core GEMs
conf.gembox 'full-core'
- conf.cc.defines = %w(MRB_ENABLE_DEBUG_HOOK)
+ conf.cc.defines += %w(MRB_ENABLE_DEBUG_HOOK)
conf.enable_test
end

Do not overwrite `conf.cc.defines`.