These steps are applicable on ARM GCC compiler. Use the following steps to use C99 coding standards with PSoC 5LP parts in Creator: -
1. Open the project in Creator in which you want to use C99 coding standards.
2. Right click on the name of project.
3. Select Build Settings.
4. Expand the Compiler node in the settings tree.
5. Select the Command Line node.
6. In the Custom Command Line property add "-std=gnu99".
Attaching the screenshot showing the steps.
After this Creator will use C99 coding standard for that partcular project.