* configure.in (DOT, DOXYGEN): use AC_CHECK_PROGS instead of
AC_CHECK_PROG which needs the third argument. [ruby-core:44433]
[Bug #6316]
* configure.in (PKG_CONFIG): fix condition to skip older version
of pkg-config. continue in backticks does not affect outside.
merge revision(s) 35387:
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_1_9_3@35388 b2dd03c8-39d4-4d8f-98ff-823fe69b080e