Enables errors for implicit function declaration by default

[lum1n0us]

Shall we?

LGTM.

Do you feel it is necessary that we should use some strict compilation flags, like -Wall -Werror, to highlight those warnings and reduce the situation where code can be compiled on some development environments, like Ubuntu, but fails on others like macOS or others?

Those are probably caused by different compilation warning flags. For example, this one will pass compilation and linking with a warning on ubuntu: warning: implicit declaration of function 'openvino_destroy'.

[ 96%] Built target iwasm
Scanning dependencies of target wasi_nn_openvino
[ 98%] Building C object CMakeFiles/wasi_nn_openvino.dir/workspaces/wamr/core/iwasm/libraries/wasi-nn/src/wasi_nn_openvino.c.o
/workspaces/wamr/core/iwasm/libraries/wasi-nn/src/wasi_nn_openvino.c: In function 'init_backend':
/workspaces/wamr/core/iwasm/libraries/wasi-nn/src/wasi_nn_openvino.c:514:5: warning: implicit declaration of function 'openvino_destroy' [-Wimplicit-function-declaration]
  514 |     openvino_destroy((void *)ov_ctx);
      |     ^~~~~~~~~~~~~~~~
[100%] Linking C shared library libwasi_nn_openvino.so
[100%] Built target wasi_nn_openvino

maybe.
implicit functions usually cause errors here on macOS.

Originally posted by @yamt in #4305 (comment)

[lum1n0us]

More issues detected by compilation flags (on development environments other than Ubuntu):

• -incompatible-pointer-type from WAMR 2.3.0 fails to build on armv7 and riscv64 (Alpine Linux, musl libc) #4277
• C2491 and C4273 from When using msvc to compile the sample in samples\basic, there is an error: "definition of dllimport function not allowed" #4280

-implicit-function-declaration and -incompatible-pointer-types could be enabled by -fpermissive or -pedantic. In an ideal situation, -pedantic should be used to perform the most strict examination and ensure compatibility. However, some existing code uses poor coding patterns to implement features, like NativeSymbol, which uses func_ptr against the rule that *ISO C forbids initialization between function pointers and ‘void **. This prevents us from applying -pedantic immediately. On the other hand, -fpermissive raises a warning indicating it is valid for C++/Objc++ but not for C. Therefore, both options are not viable.

[lum1n0us]

An observation is that GCC and Clang use different compilation options, leading to different compilation results. For example, -Wtypedef-redefinition is a Clang-specific option that can identify redefinition issues with wasm_frame_t, but GCC won't report it. Since Clang is the default compiler on Android and macOS platforms, this item will affect compatibility. Therefore, compiling on various platforms can be considered as using different compilers to check.