Compilation warnings

[Lecrapouille]

@xigh

• Missing compilation flags for Windows.
• On Linux I added some warning flags:

g++ -o src/search_async.os -c --std=c++17 -Wall -Wextra -Wuninitialized -Wundef -Wunused -Wunused-result -Wunused-parameter -Wtype-limits -Wcast-align -Wcast-qual -Wconversion -Wfloat-equal -Wpointer-arith -Wswitch-enum -Wpacked -Wold-style-cast -Wdeprecated -Wvariadic-macros -Wvla -Wsign-conversion -g -O3 -fPIC -fPIC -I. -I/home/qq/workspace_stigmee/godot/3.4.3-stable/cpp/godot-headers -I/home/qq/workspace_stigmee/godot/3.4.3-stable/cpp/include -I/home/qq/workspace_stigmee/godot/3.4.3-stable/cpp/include/core -I/home/qq/workspace_stigmee/godot/3.4.3-stable/cpp/include/gen -Isrc -I/home/qq/workspace_stigmee/godot/gdnative/stigmark/include src/search_async.cpp
src/login.cpp: In member function 'godot::String stigmee::Stigmark::login(godot::String, godot::String)':
src/login.cpp:28:46: warning: unused parameter 'p_mail' [-Wunused-parameter]
   28 |  godot::String Stigmark::login(godot::String p_mail, godot::String p_pass)
      |                                ~~~~~~~~~~~~~~^~~~~~
src/login.cpp:28:68: warning: unused parameter 'p_pass' [-Wunused-parameter]
   28 |  godot::String Stigmark::login(godot::String p_mail, godot::String p_pass)
      |                                                      ~~~~~~~~~~~~~~^~~~~~
src/search_async.cpp: In function 'void stigmee::search_async_callback(void*, int, stigmark_client_search_response_t*, int)':
src/search_async.cpp:28:52: warning: unused parameter 'status' [-Wunused-parameter]
   28 |  static void search_async_callback(void *data, int status, stigmark_client_search_response_t *res_ptr, int res_count)
      |                                                ~~~~^~~~~~
src/search_async.cpp: In member function 'void stigmee::Stigmark::search_async(godot::String)':
src/search_async.cpp:73:34: warning: use of old-style cast to 'void*' [-Wold-style-cast]
   73 |   godot::api->godot_free((void*) keyword);
      |                                  ^~~~~~~
src/search_async.cpp:73:26: warning: cast from type 'const char*' to type 'void*' casts away qualifiers [-Wcast-qual]
   73 |   godot::api->godot_free((void*) keyword);
      |                          ^~~~~~~~~~~~~~~
src/search.cpp: In member function 'godot::Array stigmee::Stigmark::search(godot::String)':
src/search.cpp:71:34: warning: use of old-style cast to 'void*' [-Wold-style-cast]
   71 |   godot::api->godot_free((void*) keyword);
      |                                  ^~~~~~~
src/search.cpp:71:26: warning: cast from type 'const char*' to type 'void*' casts away qualifiers [-Wcast-qual]
   71 |   godot::api->godot_free((void*) keyword);
      |                          ^~~~~~~~~~~~~~~
g++ -o /home/qq/workspace_stigmee/stigmee/build/libstigmark.so -shared src/gdlibrary.os src/init.os src/login.os src/search.os src/search_async.os -L/home/qq/workspace_stigmee/godot/3.4.3-stable/cpp/bin -L/home/qq/workspace_stigmee/godot/gdnative/stigmark/target/debug -lgodot-cpp.linux.release.64 -lstigmark_client