Ruby 2.1.x users (on Windows) who do "gem update" and try to build io-console 0.4.3 or later fail at link-time.
rb_sym2str was first used in console.c in r50427
rb_sym2str was implemented in ruby 2.20
Excerpt of build failure: current directory: C:/rubytest/ruby-2.1.8-x64-mingw32/lib/ruby/gems/2.1.0/gems/io-console-0.4.5 make "DESTDIR=" generating console-x64-mingw32.def compiling console.c console.c: In function 'console_key_pressed_p': console.c:722:6: warning: implicit declaration of function 'rb_sym2str' [-Wimplicit-function-declaration] linking shared-object io/console.so console.o: In function `` console_key_pressed_p': C:\rubytest\ruby-2.1.8-x64-mingw32\lib\ruby\gems\2.1.0\gems\io-console-0.4.5/console.c:722: undefined reference to ``rb_sym2str' collect2.exe: error: ld returned 1 exit status make: *** [console.so] Error 1