cmake: lib CURL_STATICLIB fixes (Windows)

- always define `CURL_STATICLIB` when building libcurl for Windows.

  This disables `__declspec(dllexport)` for exported libcurl symbols.
  In normal mode (hide symbols) these exported symbols are specified
  via `libcurl.def`. When not hiding symbols, all symbols are exported
  by default.

  Regression from 1199308dbc

  Fixes #11844

- fix to omit `libcurl.def` when not hiding private symbols.

  Regression from 2ebc74c36a

- fix `ENABLED_DEBUG=ON` + shared curl tool Windows builds by also
  omitting `libcurl.def` in this case, and exporting all symbols
  instead. This ensures that a shared curl tool can access all debug
  functions which are not normally exported from libcurl DLL.

- delete `INTERFACE_COMPILE_DEFINITIONS "CURL_STATICLIB"` for "objects"
  target.

  Follow-up to 2ebc74c36a

- delete duplicate `BUILDING_LIBCURL` definitions.

- fix `HIDES_CURL_PRIVATE_SYMBOLS` to not overwrite earlier build settings.

  Follow-up to 1199308dbc

Closes #11914

CMake/CurlSymbolHiding.cmake

@@ -26,6 +26,12 @@ include(CheckCSourceCompiles)
 option(CURL_HIDDEN_SYMBOLS "Set to ON to hide libcurl internal symbols (=hide all symbols that aren't officially external)." ON)
 mark_as_advanced(CURL_HIDDEN_SYMBOLS)
 
+if(WIN32 AND ENABLE_CURLDEBUG)
+  # We need to export internal debug functions (e.g. curl_dbg_*), so disable
+  # symbol hiding for debug builds.
+  set(CURL_HIDDEN_SYMBOLS OFF)
+endif()
+
 if(CURL_HIDDEN_SYMBOLS)
   set(SUPPORTS_SYMBOL_HIDING FALSE)