cmake: initialize variables where missing

As detected using `cmake --warn-uninitialized`.

It also lists:
- variables inherited from `Makefile.inc`, which this PR does not fix.

- a documented CMake global variable, which is unexpected:
  `CMAKE_MODULE_PATH`.
  I'd expect CMake to initialize its namespace.

- envs: `CI`, `CURL_CI` and `CURL_BUILDINFO`. Unexpected, as the manual
  mentions variables only. As of August 2024, there is no solution to
  silence them:
  https://discourse.cmake.org/t/how-to-test-for-set-env-variables-without-getting-warnings/11401

https://cmake.org/cmake/help/latest/manual/cmake.1.html#cmdoption-cmake-warn-uninitialized

Closes #16198

CMake/Macros.cmake

@@ -34,11 +34,14 @@ macro(check_include_file_concat_curl _file _variable)
   endif()
 endmacro()
 
+set(CURL_TEST_DEFINES "")  # Initialize global variable
+
 # For other curl specific tests, use this macro.
 # Return result in variable: CURL_TEST_OUTPUT
 macro(curl_internal_test _curl_test)
   if(NOT DEFINED "${_curl_test}")
     string(REPLACE ";" " " _cmake_required_definitions "${CMAKE_REQUIRED_DEFINITIONS}")
+    set(_curl_test_add_libraries "")
     if(CMAKE_REQUIRED_LIBRARIES)
       set(_curl_test_add_libraries
         "-DLINK_LIBRARIES:STRING=${CMAKE_REQUIRED_LIBRARIES}")