VMS: It seems DEC C doesn't handle certain header files quite right

With DEC C on VMS, you can use __DECC_INCLUDE_PROLOGUE.H and
__DECC_INCLUDE_EPILOGUE.H to include some DEC C specific features or
pragmas without having to touch the other header files.

It seems, however, that the current version of the compiler requires
the file names to be upcased, or it doesn't handle them quite right.

Reviewed-by: Andy Polyakov <appro@openssl.org>