Don't use gnu++03 alias of gnu++98 for ODK builds

gnu++03 is just an alias for gnu++98 and was introduced in gcc 4.8.
This prevents older compilers from building the code.

Change-Id: I7e09338f26376c08c476561cdcddde39c7e62b09
Reviewed-on: https://gerrit.libreoffice.org/11501
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>
Tested-by: Stephan Bergmann <sbergman@redhat.com>