Upgrade minimum standards for C and C++

[emasab]

C99 and C++98 are the new minimum standards.

Visual Studio 2015 is still supported and fully implements only C99.

[Copilot]

Pull Request Overview

This PR upgrades the minimum C and C++ standards from gnu90 to C99 and introduces C++98 as the minimum C++ standard. The change is motivated by the fact that C99 is fully implemented in Visual Studio 2013+ and is the default in GCC 5.1.0+, making it a reasonable baseline for modern development.

• Updated minimum C standard from gnu90 to C99
• Introduced C++98 as the minimum C++ standard
• Updated CI configuration to test against the new standards

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
CONTRIBUTING.md	Updated documentation to reflect new C99/C++98 minimum standards and removed outdated gnu90 limitations
.semaphore/semaphore.yml	Updated CI CFLAGS to use c99 and added CXXFLAGS for c++98 testing
.semaphore/semaphore-integration.yml	Updated integration CI with same compiler flag changes

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