VMS: mitigate for the C++ compiler that doesn't understand certain pragmas

This only affects __DECC_INCLUDE_EPILOGUE.H and __DECC_INCLUDE_PROLOGUE.H,
which are used automatically by HP and VSI C/C++ compilers.

Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
(Merged from https://github.com/openssl/openssl/pull/11159)