(re)allow WBMP detection with system libgd
provide getbmi and skipheader not exported functions in gd_compat.
enable WebP with system libgd when available
Merge branch 'PHP-5.5'

* PHP-5.5:
  enable WebP with system libgd when available
  (re)allow WBMP detection with system libgd provide getbmi and skipheader not exported functions in gd_compat.