Merge pull request mixxxdj#15899 from Sanjana272621/fix-15016-vcpkg-root-validation

CMake: fail early when MIXXX_VCPKG_ROOT is invalid

Author: daschuer

CMakeLists.txt

@@ -244,6 +244,19 @@ if(DEFINED ENV{MIXXX_VCPKG_ROOT} AND NOT DEFINED MIXXX_VCPKG_ROOT)
 endif()
 
 if(DEFINED MIXXX_VCPKG_ROOT)
+  # Validate MIXXX_VCPKG_ROOT points to a vcpkg root to avoid confusing follow-up errors.
+  set(_vcpkg_toolchain "${MIXXX_VCPKG_ROOT}/scripts/buildsystems/vcpkg.cmake")
+  if(NOT EXISTS "${_vcpkg_toolchain}")
+    file(TO_NATIVE_PATH "${_vcpkg_toolchain}" _vcpkg_toolchain_native)
+    message(
+      STATUS
+      "MIXXX_VCPKG_ROOT was set to \"${MIXXX_VCPKG_ROOT}\", but this is not a valid vcpkg root.\n"
+      "Expected to find: ${_vcpkg_toolchain_native}\n"
+      "Please set MIXXX_VCPKG_ROOT to the vcpkg root directory, the folder containing the vcpkg tool (vcpkg.exe on Windows)."
+    )
+    fatal_error_missing_env()
+  endif()
+
   if(
     EXISTS "${MIXXX_VCPKG_ROOT}/overlay/ports"
     OR NOT EXISTS "${MIXXX_VCPKG_ROOT}/ports"