#5 Cannot build on macOS 10.13

[FX]

On macOS 10.13, the system headers already define u_int32_t, so compilation leads to the following error:

In file included from crc32.c:20:
./crc32.h:9:30: error: typedef redefinition with different types ('unsigned long' vs 'unsigned int')
   typedef unsigned long int u_int32_t;
                             ^
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.13.sdk/usr/include/sys/_types/_u_int32_t.h:30:23: note: previous definition is here
typedef unsigned int            u_int32_t;
                                ^