common.mk: ignore timestamp directory

* common.mk (clean-ext): ignore error on removing non-existent
  timestamp directory.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47126 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
configure.in: duplicated linker flags

* configure.in (with_jemalloc): get rid of duplicated linker flags,
  as AC_CHECK_LIB adds it.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47127 b2dd03c8-39d4-4d8f-98ff-823fe69b080e