configure.in, missing.h: jemalloc mangling

* configure.in (with-jemalloc): also check for header, for ABIs
  which JEMALLOC_MANGLE is needed, i.e., Mach-O and PE-COFF
  platforms.  [ruby-core:62939] [Feature #9113]
* include/ruby/missing.h: include alternative malloc header to
  replace memory management functions.
* dln.c, io.c, parse.y, st.c: undef malloc family before
  re-definition to suppress warnings.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46354 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
version.c: show malloc_conf

* configure.in (jemalloc): check for the header regardless drop-in
  libjemalloc is found, for `malloc_conf` declaration.
* version.c (ruby_show_version): show `malloc_conf` if set.
  [Feature #9113]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46355 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
remove dead links