[vcpkg.cmake] Fix CMP0126 warnings (#23784)

When running CMake 3.21 or newer in trace or debug mode, vcpkg.cmake
would trigger the CMP0126 warning for VCPKG_MANIFEST_DIR and
VCPKG_INSTALLED_DIR.

The regular variable was used to set the initial value of the cache
variable of the same name. This patch adds the regular variables
Z_VCPKG_MANIFEST_DIR_INITIAL_VALUE and
Z_VCPKG_INSTALLED_DIR_INITIAL_VALUE that are used to initialize their
respective cache variables.

scripts/buildsystems/vcpkg.cmake

@@ -56,12 +56,13 @@ if(VCPKG_PREFER_SYSTEM_LIBS)
 endif()
 
 # Manifest options and settings
+set(Z_VCPKG_MANIFEST_DIR_INITIAL_VALUE "${VCPKG_MANIFEST_DIR}")
 if(NOT DEFINED VCPKG_MANIFEST_DIR)
     if(EXISTS "${CMAKE_SOURCE_DIR}/vcpkg.json")
-        set(VCPKG_MANIFEST_DIR "${CMAKE_SOURCE_DIR}")
+        set(Z_VCPKG_MANIFEST_DIR_INITIAL_VALUE "${CMAKE_SOURCE_DIR}")
     endif()
 endif()
-set(VCPKG_MANIFEST_DIR "${VCPKG_MANIFEST_DIR}"
+set(VCPKG_MANIFEST_DIR "${Z_VCPKG_MANIFEST_DIR_INITIAL_VALUE}"
     CACHE PATH "The path to the vcpkg manifest directory." FORCE)
 
 if(DEFINED VCPKG_MANIFEST_DIR AND NOT VCPKG_MANIFEST_DIR STREQUAL "")
@@ -386,16 +387,16 @@ if(NOT Z_VCPKG_ROOT_DIR)
 endif()
 
 if(DEFINED VCPKG_INSTALLED_DIR)
-    # do nothing
+    set(Z_VCPKG_INSTALLED_DIR_INITIAL_VALUE "${VCPKG_INSTALLED_DIR}")
 elseif(DEFINED _VCPKG_INSTALLED_DIR)
-    set(VCPKG_INSTALLED_DIR "${_VCPKG_INSTALLED_DIR}")
+    set(Z_VCPKG_INSTALLED_DIR_INITIAL_VALUE "${_VCPKG_INSTALLED_DIR}")
 elseif(VCPKG_MANIFEST_MODE)
-    set(VCPKG_INSTALLED_DIR "${CMAKE_BINARY_DIR}/vcpkg_installed")
+    set(Z_VCPKG_INSTALLED_DIR_INITIAL_VALUE "${CMAKE_BINARY_DIR}/vcpkg_installed")
 else()
-    set(VCPKG_INSTALLED_DIR "${Z_VCPKG_ROOT_DIR}/installed")
+    set(Z_VCPKG_INSTALLED_DIR_INITIAL_VALUE "${Z_VCPKG_ROOT_DIR}/installed")
 endif()
 
-set(VCPKG_INSTALLED_DIR "${VCPKG_INSTALLED_DIR}"
+set(VCPKG_INSTALLED_DIR "${Z_VCPKG_INSTALLED_DIR_INITIAL_VALUE}"
     CACHE PATH
     "The directory which contains the installed libraries for each triplet" FORCE)
 set(_VCPKG_INSTALLED_DIR "${VCPKG_INSTALLED_DIR}"