Concentrate compiler dependencies to compiler.h; make sure compiler.h
is included first in every .c file (since some prototypes may depend
on the presence of feature request macros.)
Actually use the conditional inclusion of various functions (totally
broken in previous releases.)
Add special operators to allow the use of floating-point constants in
contexts other than DW/DD/DQ/DT/DO.
As part of this checkin, make MAX_KEYWORD generated by tokhash.pl,
since it knows what all the keywords are so it can tell which one is
the longest.