[cmake] Use GNUInstallDirs to support custom installation dirs.

This is the original patch in my GNUInstallDirs series, now last to merge as the final piece!

It arose as a new draft of D28234. I initially did the unorthodox thing of pushing to that when I wasn't the original author, but since I ended up

 - Using `GNUInstallDirs`, rather than mimicking it, as the original author was hesitant to do but others requested.

 - Converting all the packages, not just LLVM, effecting many more projects than LLVM itself.

I figured it was time to make a new revision.

I have used this patch series (and many back-ports) as the basis of NixOS/nixpkgs#111487 for my distro (NixOS), which was merged last spring (2021). It looked like people were generally on board in D28234, but I make note of this here in case extra motivation is useful.

---

As pointed out in the original issue, a central tension is that LLVM already has some partial support for these sorts of things. Variables like `COMPILER_RT_INSTALL_PATH` have already been dealt with. Variables like `LLVM_LIBDIR_SUFFIX` however, will require further work, so that we may use `CMAKE_INSTALL_LIBDIR`.

These remaining items will be addressed in further patches. What is here is now rote and so we should get it out of the way before dealing more intricately with the remainder.

Reviewed By: #libunwind, #libc, #libc_abi, compnerd

Differential Revision: https://reviews.llvm.org/D99484

CMakeLists.txt

@@ -10,6 +10,8 @@ endif()
 #===============================================================================
 cmake_minimum_required(VERSION 3.13.4)
 
+include(GNUInstallDirs)
+
 set(LLVM_COMMON_CMAKE_UTILS "${CMAKE_CURRENT_SOURCE_DIR}/../cmake")
 
 # Add path for custom modules
@@ -415,9 +417,9 @@ endif ()
 # TODO: Projects that depend on libc++ should use LIBCXX_GENERATED_INCLUDE_DIR
 # instead of hard-coding include/c++/v1.
 
-set(LIBCXX_INSTALL_INCLUDE_DIR "include/c++/v1" CACHE PATH
+set(LIBCXX_INSTALL_INCLUDE_DIR "${CMAKE_INSTALL_INCLUDEDIR}/c++/v1" CACHE PATH
     "Path where target-agnostic libc++ headers should be installed.")
-set(LIBCXX_INSTALL_RUNTIME_DIR bin CACHE PATH
+set(LIBCXX_INSTALL_RUNTIME_DIR "${CMAKE_INSTALL_BINDIR}" CACHE PATH
     "Path where built libc++ runtime libraries should be installed.")
 
 if(LLVM_ENABLE_PER_TARGET_RUNTIME_DIR AND NOT APPLE)
@@ -426,7 +428,7 @@ if(LLVM_ENABLE_PER_TARGET_RUNTIME_DIR AND NOT APPLE)
   set(LIBCXX_GENERATED_INCLUDE_TARGET_DIR "${LLVM_BINARY_DIR}/include/${LLVM_DEFAULT_TARGET_TRIPLE}/c++/v1")
   set(LIBCXX_INSTALL_LIBRARY_DIR lib${LLVM_LIBDIR_SUFFIX}/${LLVM_DEFAULT_TARGET_TRIPLE} CACHE PATH
       "Path where built libc++ libraries should be installed.")
-  set(LIBCXX_INSTALL_INCLUDE_TARGET_DIR "include/${LLVM_DEFAULT_TARGET_TRIPLE}/c++/v1" CACHE PATH
+  set(LIBCXX_INSTALL_INCLUDE_TARGET_DIR "${CMAKE_INSTALL_INCLUDEDIR}/${LLVM_DEFAULT_TARGET_TRIPLE}/c++/v1" CACHE PATH
       "Path where target-specific libc++ headers should be installed.")
   if(LIBCXX_LIBDIR_SUBDIR)
     string(APPEND LIBCXX_LIBRARY_DIR /${LIBCXX_LIBDIR_SUBDIR})

cmake/Modules/HandleLibCXXABI.cmake

@@ -1,8 +1,9 @@
-
 #===============================================================================
 # Add an ABI library if appropriate
 #===============================================================================
 
+include(GNUInstallDirs)
+
 #
 # _setup_abi: Set up the build to use an ABI library
 #
@@ -63,7 +64,7 @@ macro(setup_abi_lib abidefines abishared abistatic abifiles abidirs)
 
         if (LIBCXX_INSTALL_HEADERS)
           install(FILES "${LIBCXX_BINARY_INCLUDE_DIR}/${fpath}"
-            DESTINATION include/c++/v1/${dstdir}
+            DESTINATION "${CMAKE_INSTALL_INCLUDEDIR}/c++/v1/${dstdir}"
             COMPONENT cxx-headers
             PERMISSIONS OWNER_READ OWNER_WRITE GROUP_READ WORLD_READ
             )