[llvm][cmake] Follow up to D117973

1. Slightly document the "mark advanced" variable used to control the
   installed CMake package dir.

   I would document it more, but I am considering in the future adding
   pkg-config support in this manner, after which `_PACKGE_DIR` is
   probably better called `_CMAKE_PACKGE_DIR` or similar.

2. Convey the custom path to the legacy `llvm-config` binary.

Reviewed By: sebastian-ne

Differential Revision: https://reviews.llvm.org/D130539

1 files changed, 5 insertions, 1 deletions

diff --git a/llvm/tools/llvm-config/llvm-config.cpp b/llvm/tools/llvm-config/llvm-config.cpp
index e9fa7bd..2c6c55f 100644
--- a/llvm/tools/llvm-config/llvm-config.cpp
+++ b/llvm/tools/llvm-config/llvm-config.cpp
@@ -370,7 +370,11 @@ int main(int argc, char **argv) {
       ActiveBinDir = std::string(Path.str());
     }
     ActiveLibDir = ActivePrefix + "/lib" + LLVM_LIBDIR_SUFFIX;
-    ActiveCMakeDir = ActiveLibDir + "/cmake/llvm";
+    {
+      SmallString<256> Path(LLVM_INSTALL_PACKAGE_DIR);
+      sys::fs::make_absolute(ActivePrefix, Path);
+      ActiveCMakeDir = std::string(Path.str());
+    }
     ActiveIncludeOption = "-I" + ActiveIncludeDir;
   }