gmake.mk: no test-knownbug if unnecessary

* defs/gmake.mk (test-all): no needs to depend on test-knownbug
  always.  it is included in TEST_TARGETS if it is given.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39957 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
mkmf.rb: force refererence in MAIN_DOES_NOTHING

* lib/mkmf.rb (MAIN_DOES_NOTHING): force to refer symbols for tests
  to be preserved.  [ruby-core:53745] [Bug #8169]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39958 b2dd03c8-39d4-4d8f-98ff-823fe69b080e