./configure --with-platform=contrib/platform/$name will apply all patches
found in contrib/platform/$name.patches/ directory during "configure" phase.
It allows vendors to distribute not-accepted or vendor-specific patches as
part of standard ompi tarball and reduce burden of keeping private repos.
Added:
* VPATH support
* $srcdir/.platform_patches file created if patches already applied
* configure will fail with error if patch cannot be applied (using dry-run)
* configure will fail if patches modifying *.[am,m4] files (unsupported)
* remove unused patch_errors var
* set with_ident_string if platform file applied patches
