“#include” a text file in a C program as a char[]

I’d suggest using (unix util)xxd for this.
you can use it like so

$ echo hello world > a
$ xxd -i a

outputs:

unsigned char a[] = {
  0x68, 0x65, 0x6c, 0x6c, 0x6f, 0x20, 0x77, 0x6f, 0x72, 0x6c, 0x64, 0x0a
};
unsigned int a_len = 12;