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.