php53: fix on darwin

* add libssh2 to build inputs (configure fails without this)
* link with stdc++
* add icu/lib directory to DYLD_LIBRARY_PATH (and wrap binaries with this
  environment variable
* add libiconv to build inputs
* fix --with-iconv configure flag (was --with-iconv-dir which is not a valid
  flag)
php53: add optional pcntl flag
php53: restore nix gd version
Merge pull request #650 from lovek323/php53

php53: fix on darwin