Avoid library header include in crc32.c for Z_SOLO.

Avoid library header include in crc32.c for Z_SOLO.

crc32.c was #including limits.h in order to find a four-byte integer
type.  It was doing this even if Z_SOLO were defined, violating the
intent of Z_SOLO, which is to include no library headers and require
no library functions.  Now crc32.c obeys the intent of Z_SOLO, but
with the downside that crc32() will be slower than when not compiled
with Z_SOLO.  This can be remedied manually by typedefing u4 to a
known four-byte unsigned integer type, and #defining BYFOUR in
crc32.c.