Tried to build version 8.6.16 with GCC 4.2 & llvm-gcc from Xcode, same error with both compilers, though the number of the variable changes.

In function ‘-[TKContentView(TKWindowEvent) initWithFrame:]’:
error: assignment of read-only variable ‘prop.238’
make: *** [tkMacOSXWindowEvent.o] Error 1