If one has an existing cmake cache and adds a vcpkg.json manifest file and then set VCPKG_MANIFEST_MODE to true
the manifest file will not be found since the _VCPKG_MANIFEST_DIR internal cache variable was already set and will not be updated. Hence the manifest file it not found and an error is printed

the cache variable it set here

and this check prevents it from being updated

Environment

• OS: Windows
• CMake: 3.17.2

To Reproduce
Steps to reproduce the behavior:

1. run cmake configure on a project without a manifest file
2. add a manifest file
3. set VCPKG_MANIFEST_MODE to true in cmake
4. run configure again

Expected behavior
The newly added manifest file should be found
Or an error saying the one has to clear the cache to make it work

Failure logs

CMake Error at C:/Users/petst55.AD/Documents/vcpkg/vcpkg/scripts/buildsystems/vcpkg.cmake:41 (message):
  vcpkg manifest mode was enabled, but we couldn't find a manifest file
  (vcpkg.json) in any directories above
  C:/Users/petst55.AD/Documents/Inviwo/sgct/sgct.  Please add a manifest, or
  disable manifests by turning off VCPKG_MANIFEST_MODE.
Call Stack (most recent call first):
  C:/Users/petst55.AD/Documents/Inviwo/sgct/build/CMakeFiles/3.17.2/CMakeSystem.cmake:6 (include)
  CMakeLists.txt:10 (project)

Configuring incomplete, errors occurred!
See also "C:/Users/petst55.AD/Documents/Inviwo/sgct/build/CMakeFiles/CMakeOutput.log".
See also "C:/Users/petst55.AD/Documents/Inviwo/sgct/build/CMakeFiles/CMakeError.log".

FYI @strega-nil