Copy of the CRC and checksum functions from c.snippets.org (site no longer exists, but a backup can be found on archive.org).
I ceaned the code up a tiny bit so it compiles with gcc -Wall -pedantic -std=c99.
I added missing return type declarations and replaced sniptype.h, BYTE, WORD,
DWORD, Boolean_T etc. with stdint.h, stdbool.h, uint8_t, uint16_t,
uint32_t, bool etc. These changes require a C99 compatible compiler. This
should also make it 64bit compatible.