Fix Clang support by deleting -multiply-definedsuppress flag from DLDFLAGS

`-multiply_definedsuppress` flag breaks the build on the newest Clang (which comes with OS X Mavericks).

Here is the error that happens:

```
linking shared-object puma/puma_http11.bundle
clang: error: unknown argument: '-multiply_definedsuppress' [-Wunused-command-line-argument-hard-error-in-future]
clang: note: this will be a hard error (cannot be downgraded to a warning) in the future
make: *** [puma_http11.bundle] Error 1
```

My version of `clang` is:

```
Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr --with-gxx-include-dir=/usr/include/c++/4.2.1
Apple LLVM version 5.1 (clang-503.0.38) (based on LLVM 3.4svn)
Target: x86_64-apple-darwin13.1.0
Thread model: posix
```

This commit tells `extconf.rb` to delete this flag from DLDFLAGS variable in Makefile.