merge revision(s) 39214,39221: [Backport #7830]

	configure.in: Werror-implicit-function-declaration

	* configure.in (warnflags): -Werror-implicit-function-declaration
  haven't been used as-is, but always replaced with -Werror= or -W.
	* configure.in (warnflags): disable -Werror by default unless
	  development.  [ruby-core:52131] [Bug #7830]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_0_0@39564 b2dd03c8-39d4-4d8f-98ff-823fe69b080e