--C99 option

This specifies that the code is compiled to C99 standard:
https://developer.arm.com/documentation/dui0472/m/Compiler-Command-line-Options/--c99

I note you are using Arm Compiler 5, which has long been superseded by Arm Compiler for Embedded (currently at version 6.18). There is a migration guide here:
https://developer.arm.com/documentation/100068

This specifies that the code is compiled to C99 standard:
https://developer.arm.com/documentation/dui0472/m/Compiler-Command-line-Options/--c99

I note you are using Arm Compiler 5, which has long been superseded by Arm Compiler for Embedded (currently at version 6.18). There is a migration guide here:
https://developer.arm.com/documentation/100068