extconf.rb: remove "-Wl,--no-undefined" from ldflags

Remove ldflag "-Wl,--no-undefined" if existent to allow for loading
libyajl with dlopen.

This fixes building the native extensions on systems like gentoo which
set the ldflag "-Wl,--no-undefined" by default.
Merge pull request #11 from lebrinkma/fix-gentoo

extconf.rb: remove "-Wl,--no-undefined" from ldflags