Fix that LLVM currently rebuilds unnecessarily when git HEAD changes.

This removes the file dependency between LLVM and the git HEAD file,
which currently causes that we rebuild LLVM/Cling/Dictionaries
once someone does anything meaningful in git.

interpreter/llvm/src/include/llvm/Support/CMakeLists.txt

@@ -38,6 +38,13 @@ set(version_inc "${CMAKE_CURRENT_BINARY_DIR}/VCSRevision.h")
 
 set(get_svn_script "${LLVM_CMAKE_PATH}/GenerateVersionFromCVS.cmake")
 
+# Ugly hack to prevent rebuilding LLVM whenever the git HEAD timestamp
+# changes. This is properly solved by setting LLVM_APPEND_VC_REV to OFF
+# but this only really works once this review is included in our LLVM
+# version: https://reviews.llvm.org/D35377
+# Once our LLVM version includes this review, this code can be removed
+# as we properly set LLVM_APPEND_VC_REV in interpreter/CMakeLists.txt.
+set(llvm_vc)
 if(DEFINED llvm_vc)
   # Create custom target to generate the VC revision include.
   add_custom_command(OUTPUT "${version_inc}"