Do not use a libdir for glibc, it breaks Linux PPC64 (#70)

Fixes [Bug #12666]

Author: jeremyevans

test/fiddle/helper.rb

@@ -47,8 +47,8 @@
  libm_so = libc_so
  else
  # glibc
- libc_so = File.join(libdir, "libc.so.6")
- libm_so = File.join(libdir, "libm.so.6")
+ libc_so = "libc.so.6"
+ libm_so = "libm.so.6"
  end
 when /mingw/, /mswin/
  require "rbconfig"