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.)
Use the same crc64 that we already use for the symbol table hash as
the perfect hash function prehash. We appear to get radically faster
convergence this way, and the crc64 is probably *faster*, since the
table likely to be resident in memory.
Combining arithmetric (add) and bitwise (xor) mixing seems to give
better result than either.
With the new prehash function, we find a valid hash much quicker.
Speed up pptok.c by just doing |= 0x20 instead of calling tolower() for
every character during prehashing. This is good enough for our needs,
since we don't have any tokens containing the characters @ [ \ ] _ nor
any high-bit characters (in which case we'd have to worry about multibyte
anyway.)
