UNDERFLOW keyword with gcc on MSYS2

I ran into some really weird problem with the compilation of a very simple code

typedef volatile struct _underflow_test
{
    unsigned int    OPERATION_MODE;
    unsigned int    UNDERFLOW;
} underflow_test;

This code fails to compile in MSYS2 using gcc12.2. Using the -E option I have checked that the code about is converted to this:

typedef volatile struct _underflow_test
{
    unsigned int OPERATION_MODE;
    unsigned int 
# 4137 "C:/Users/work/eec_control/TC367_project/Libraries/Infra/Sfr/TC36A/_Reg/IfxGeth_regdef.h" 3
                      4
# 4137 "C:/Users/work/eec_control/TC367_project/Libraries/Infra/Sfr/TC36A/_Reg/IfxGeth_regdef.h"
                               ;
} underflow_test;

So preprocessor converted UNDERFLOW into just 4. Then this conversion fails to compile. Using the gcc11 doesn't do this - it doesn't convert UNDERFLOW field into 4.

The command line is the same in both cases:

cd C:/Users/work/build-eec-gnu/core_files/bsp && C:/msys64/mingw64/bin/gcc.exe 
-DROOT_INSTALL_DIR="\"C:/Program Files (x86)/eec-control\"" 
@CMakeFiles/bsp-core-tc367-a1.dir/includes_C.rsp 
-Wall -Wextra -Wno-unknown-pragmas -Wfloat-equal -Wconversion -Wparentheses -Wunused-parameter -Wunused-variable -Wstrict-prototypes  
-DMAJOR_RELEASE_NUMBER=1 -DMINOR_RELEASE_NUMBER=0 -DPATCH_RELEASE_NUMBER=42 -E -O0 -DNDEBUG -g 
-fdiagnostics-color=always -std=gnu99 -MD -MT core_files/bsp/CMakeFiles/bsp-core-tc367-a1.dir/hal.c.obj 
-MF CMakeFiles/bsp-core-tc367-a1.dir/hal.c.obj.d -o CMakeFiles/bsp-core-tc367-a1.dir/hal.c.obj 
-c C:/Users/work/eec_control/core_files/bsp/hal.c 

Anyone know what could be causing this and how to solve this strange effect? Perhaps I'm missing something.

Kudos to Ian Abbot for the nudge to the right direction. It was caused by the #define collision between the common code and my supplier's code. The supplier's code expected strict ANSI code without GNU extensions.

The fix is to set the CMake extensions flag using set(CMAKE_C_EXTENSIONS OFF).