Fix conflicting C function definitions (rust-lang#1812)

Declare the function signatures instead of importing system definitions. This avoids conflict between our pre-build libraries and the C files that are built on the user machine.

Also remove the gen_c_lib.c file which seems to be dead code.

Author: celinval

library/kani/gen_c_lib.c

@@ -1,12 +1,15 @@
 // Copyright Kani Contributors
 // SPDX-License-Identifier: Apache-2.0 OR MIT
-
-#include <assert.h>
-#include <stdbool.h>
 #include <stddef.h>
 #include <stdint.h>
-#include <stdlib.h>
-#include <string.h>
+
+// Declare functions instead of importing more headers in order to avoid conflicting definitions.
+// See https://github.com/model-checking/kani/issues/1774 for more details.
+void  free(void *ptr);
+void *memcpy(void *dst, const void *src, size_t n);
+void *calloc(size_t nmemb, size_t size);
+
+typedef __CPROVER_bool bool;
 
 // `assert` then `assume`
 #define __KANI_assert(cond, msg)            \