Also set the CXXFLAG to the user supplied flags

Reviewed-by: NRich Salz <rsalz@openssl.org>

GH: #2025

Configure

@@ -1305,7 +1305,7 @@ unless ($disabled{"crypto-mdebug-backtrace"})
 		}
 	}
 
-if ($user_cflags ne "") { $config{cflags}="$config{cflags}$user_cflags"; }
+if ($user_cflags ne "") { $config{cflags}="$config{cflags}$user_cflags"; $config{cxxflags}="$config{cxxflags}$user_cflags";}
 else                    { $no_user_cflags=1;  }
 if (@user_defines) { $config{defines}=[ @{$config{defines}}, @user_defines ]; }
 else               { $no_user_defines=1;    }