Such environments are missing void type, older K&R C, and
such supports already had been removed in various places.
Furthermore, current hardcoded 'SIGNALRETURNSINT' is wrong.
Some files were trying to access the global includes directy in the
build area with things like #include <api/c/tt_c.h>, which is now
wrong. Se we fix all of those up. tt_c.h and tttk.h are now global,
so we no longer need to root around various build dirs to find them.
TT builds again.
OBATA Akio