Use modern way to set the C/CXX standad
Summary:
This allows later modification of the selected standard.
Raise C from C89 to C90 as C89 is not supported by the CMAKE flag
https://cmake.org/cmake/help/v3.16/prop_tgt/C_STANDARD.html#prop_tgt:C_STANDARD
Subscribers: kde-frameworks-devel, kde-buildsystem
Tags: Frameworks, Build System
Differential Revision: https://phabricator.kde.org/D24841