configure: refuse to build with jemalloc when header is missing

	[ruby-core:90964] [Bug #15520]

	Freom: Misty De Meo <mistydemeo@github.com>

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_5@67240 b2dd03c8-39d4-4d8f-98ff-823fe69b080e