lang/ruby2[4|5]: Use internal RUBY_DLDFLAGS right way, get rid of unescaped macro '$@' in the pkg-config template

lang/ruby23 is already fixed same way, port it to newer releases

By the way, this unbreak ninja builds with any port relied on libruby.so

PR: 229898
Submitted by: fluffy
Reviewed by: ruby (miwi)
Approved by: ruby (miwi)
Exp-run by: antoine
MFH: 2018Q3
Differential Revision: D16341