Using OpenGL in Swift project

I'm trying to port one of my apps over to use Swift. It contains an OpenGL draw loop so I've copied the original .m file into my new project and added a *-Bridging-Header.h file. I've also added a build phase to link with the OpenGL.framework (although I'm not sure I needed to and it made no difference to the issue).

Originally I borrowed heavily from Apple's example OpneGL project and within one of the files I'm trying to compile there is:

#include "sourceUtil.h"

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#import <OpenGL/OpenGL.h>

demoSource* srcLoadSource(const char* filepathname)
{
    demoSource* source = (demoSource*) calloc(sizeof(demoSource), 1);
   
    // Check the file name suffix to determine what type of shader this is
    const char* suffixBegin = filepathname + strlen(filepathname) - 4;
   
    if(0 == strncmp(suffixBegin, ".fsh", 4))
    {
        source->shaderType = GL_FRAGMENT_SHADER;
    }
    else if(0 == strncmp(suffixBegin, ".vsh", 4))
    {
        source->shaderType = GL_VERTEX_SHADER;
    }
    else
    {
        // Unknown suffix
        source->shaderType = 0;
    }
// more code follows
.
}

However, GL_FRAGMENT_SHADER is causing Xcode to stop any build with the error "Use of undeclared identifier 'GL_FRAGMENT_SHADER'" - similarly with the GL_VERTEX_SHADER. I presume there'll be more errors but currently this is what Xcode stops at, along with some warnings.