Use TEA, but perhaps also see "Hello World as a C extension".
aricb I hope someone will fill in a few more details here. Some questions I have are:
What are the popular compilers used for Tcl extensions on OS X? What Mac-specific compiler arguments should an extension writer be familiar with?
The MacOS X page mentions that Tk can be compiled to have a native Aqua look and feel, or it can be compiled to use X-Windows. Does an extension writer need to worry about this distinction, and if so, what does one do differently in each case?