tlshd: configure.ac: Use AC_CHECK_HEADER instead of AC_CHECK_FILE

[alistair23]

AC_CHECK_FILE doesn't work when cross-compiling [1], which breaks a lot of use cases (like OpenEmbedded for one). Autoconf supports AC_CHECK_HEADER which can be used instead and is more robust [2] so let's use that instead.

1: https://www.gnu.org/software/autoconf/manual/autoconf-2.68/html_node/Files.html 2: https://www.gnu.org/software/autoconf/manual/autoconf-2.69/html_node/Generic-Headers.html

[oracle-contributor-agreement[bot]]

Thank you for your pull request and welcome to our community! To contribute, please sign the Oracle Contributor Agreement (OCA). The following contributors of this PR have not signed the OCA:

• PR author: [email protected] (@alistair23)
• [email protected] (@alistair23)

To sign the OCA, please create an Oracle account and sign the OCA in Oracle's Contributor Agreement Application.

When signing the OCA, please provide your GitHub username. After signing the OCA and getting an OCA approval from Oracle, this PR will be automatically updated.

If you are an Oracle employee, please make sure that you are a member of the main Oracle GitHub organization, and your membership in this organization is public.