extconf.rb: no empty CPP

* ext/fiddle/extconf.rb: pass CPP only if necessary, should not
  make it empty.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48933 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ext/fiddle/extconf.rb

@@ -65,11 +65,11 @@ begin
     --enable-builddir=#{RUBY_PLATFORM}
   ]
   args << libffi.opt if libffi.opt
+  args << "CPP=#{cpp}" if cpp
   args.concat %W[
       CC=#{cc} CFLAGS=#{libffi.cflags}
       CXX=#{cxx} CXXFLAGS=#{RbConfig.expand("$(CXXFLAGS)", nowarn)}
       LD=#{ld} LDFLAGS=#{libffi.ldflags}
-      CPP=#{cpp}
   ]
 
   FileUtils.rm_f("#{libffi.include}/ffitarget.h")