cmake: allow disabling building with the install name directory

When custom install names and rpaths setups are used they may not
work in the build tree as-is (namely when using absolute paths for
install names in order to avoid rpath juggling in downstream projects).

Add a flag for opting out of this behaviour.

See: https://reviews.llvm.org/D42463

Author: mathstuf

llvm/CMakeLists.txt

@@ -6,7 +6,14 @@ set(LLVM_COMMON_CMAKE_UTILS ${CMAKE_CURRENT_SOURCE_DIR}/../cmake)
 include(${LLVM_COMMON_CMAKE_UTILS}/Modules/CMakePolicy.cmake
   NO_POLICY_SCOPE)
 
-set(CMAKE_BUILD_WITH_INSTALL_NAME_DIR ON)
+# Builds with custom install names and installation rpath setups may not work
+# in the build tree. Allow these cases to use CMake's default build tree
+# behavior by setting `LLVM_NO_INSTALL_NAME_DIR_FOR_BUILD_TREE` to do this.
+option(LLVM_NO_INSTALL_NAME_DIR_FOR_BUILD_TREE "If set, use CMake's default build tree install name directory logic (Darwin only)" OFF)
+mark_as_advanced(LLVM_NO_INSTALL_NAME_DIR_FOR_BUILD_TREE)
+if(NOT LLVM_NO_INSTALL_NAME_DIR_FOR_BUILD_TREE)
+  set(CMAKE_BUILD_WITH_INSTALL_NAME_DIR ON)
+endif()
 
 if(NOT DEFINED LLVM_VERSION_MAJOR)
   set(LLVM_VERSION_MAJOR 18)

llvm/docs/CMake.rst

@@ -733,6 +733,13 @@ enabled sub-projects. Nearly all of these variable names begin with
   directory contains executables with the expected names, no separate
   native versions of those executables will be built.
 
+**LLVM_NO_INSTALL_NAME_DIR_FOR_BUILD_TREE**:BOOL
+  Defaults to ``OFF``. If set to ``ON``, CMake's default logic for library IDs
+  on Darwin in the build tree will be used. Otherwise the install-time library
+  IDs will be used in the build tree as well. Mainly useful when other CMake
+  library ID control variables (e.g., ``CMAKE_INSTALL_NAME_DIR``) are being
+  set to non-standard values.
+
 **LLVM_OPTIMIZED_TABLEGEN**:BOOL
   If enabled and building a debug or asserts build the CMake build system will
   generate a Release build tree to build a fully optimized tablegen for use