build: fix Codacy/CppCheck warnings

- remove unused variables
- declare conditionally used variables conditionally
- suppress unused variable warnings in the CMake tests
- remove dead variable stores
- consistently use WIN32 macro to detect Windows

Closes https://github.com/curl/curl/pull/3739

src/tool_doswin.c

@@ -599,7 +599,6 @@ SANITIZEcode rename_if_reserved_dos_device_name(char **const sanitized,
       }
       memmove(base + 1, base, blen + 1);
       base[0] = '_';
-      ++blen;
     }
   }
 #endif