Use CMAKE_INSTALL_PREFIX for setting LLVM_INSTALL_PREFIX

[opoplawski]

Summary

Using -get_filename_component() to compute the install prefix is fragile and subject to breakage in the presence of symbolic links. We know the install prefix via CMAKE_INSTALL_PREFIX, so use it.

This came out of part of my work testing parallel installable versions of llvm/clang for Fedora, and looking to be able to have clang's CMakeLists.txt be able to do:

find_package(LLVM 3.9 REQUIRED)
list(APPEND CMAKE_MODULE_PATH "${LLVM_CMAKE_DIR}")

to locate llvm.

cmake/modules/CMakeLists.txt

 
 # Generate LLVMConfig.cmake for the install tree.
 set(LLVM_CONFIG_CODE "
-# Compute the installation prefix from this LLVMConfig.cmake file location.
-get_filename_component(LLVM_INSTALL_PREFIX \"\${CMAKE_CURRENT_LIST_FILE}\" PATH)")
-# Construct the proper number of get_filename_component(... PATH)
-# calls to compute the installation prefix.
-string(REGEX REPLACE "/" ";" _count "${LLVM_INSTALL_PACKAGE_DIR}")
-foreach(p ${_count})
-  set(LLVM_CONFIG_CODE "${LLVM_CONFIG_CODE}
-get_filename_component(LLVM_INSTALL_PREFIX \"\${LLVM_INSTALL_PREFIX}\" PATH)")
-endforeach(p)
+set(LLVM_INSTALL_PREFIX \"${CMAKE_INSTALL_PREFIX}\")")
 set(LLVM_CONFIG_INCLUDE_DIRS "\${LLVM_INSTALL_PREFIX}/include")
 set(LLVM_CONFIG_LIBRARY_DIRS "\${LLVM_INSTALL_PREFIX}/lib\${LLVM_LIBDIR_SUFFIX}")
 set(LLVM_CONFIG_CMAKE_DIR "\${LLVM_INSTALL_PREFIX}/${LLVM_INSTALL_PACKAGE_DIR}")