The SNIPPETS C Source Code Archive has a CRC32 implementation that is freely usable:
/* Copyright (C) 1986 Gary S. Brown. You may use this program, or
code or tables extracted from it, as desired without restriction.*/
(Unfortunately, c.snippets.org seems to have died. Fortunately, the Wayback Machine has it archived.)
In order to be able to compile the code, you’ll need to add typedefs for BYTE as an unsigned 8-bit integer and DWORD as an unsigned 32-bit integer, along with the header files crc.h & sniptype.h.
The only critical item in the header is this macro (which could just as easily go in CRC_32.c itself:
#define UPDC32(octet, crc) (crc_32_tab[((crc) ^ (octet)) & 0xff] ^ ((crc) >> 8))