[polly][cmake] Use `GNUInstallDirs` to support custom installation dirs

[Ericson2314]

Summary

I am breaking apart D99484 so the cause of build failures is easier to
understand.

[Meinersbur]

AFAICS, the commit did not break any Polly buildbots (https://lab.llvm.org/buildbot/#/changes/44169). That is, the Polly part looks good to me.

polly/CMakeLists.txt

+include(GNUInstallDirs)
+
 # Check if this is a in tree build.
 if (NOT DEFINED LLVM_MAIN_SRC_DIR)
   project(Polly)
   cmake_minimum_required(VERSION 3.13.4)
 
   # Where is LLVM installed?
   find_package(LLVM CONFIG REQUIRED)
   set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} ${LLVM_CMAKE_DIR})
include_directories(
   ${ISL_INCLUDE_DIRS}
   ${CMAKE_CURRENT_SOURCE_DIR}/lib/External/pet/include
   ${CMAKE_CURRENT_SOURCE_DIR}/lib/External
   ${CMAKE_CURRENT_BINARY_DIR}/include
   )
 
 if (NOT LLVM_INSTALL_TOOLCHAIN_ONLY)
   install(DIRECTORY include/
-    DESTINATION include
+    DESTINATION "${CMAKE_INSTALL_INCLUDEDIR}"
     FILES_MATCHING
     PATTERN "*.h"
     )
 
   install(DIRECTORY ${POLLY_BINARY_DIR}/include/
-    DESTINATION include
+    DESTINATION "${CMAKE_INSTALL_INCLUDEDIR}"
     FILES_MATCHING
     PATTERN "*.h"
     PATTERN "CMakeFiles" EXCLUDE
     )
 endif()
 
 add_definitions( -D_GNU_SOURCE )
 

polly/cmake/CMakeLists.txt

 # Keep this in sync with llvm/cmake/CMakeLists.txt!
 
+include(ExtendPath)
 include(FindPrefixFromConfig)
 
 set(LLVM_INSTALL_PACKAGE_DIR "lib${LLVM_LIBDIR_SUFFIX}/cmake/llvm")
 set(POLLY_INSTALL_PACKAGE_DIR "lib${LLVM_LIBDIR_SUFFIX}/cmake/polly")
 if (CMAKE_CONFIGURATION_TYPES)
   set(POLLY_EXPORTS_FILE_NAME "PollyExports-$<LOWER_CASE:$<CONFIG>>.cmake")
 else()
   # avoid conflicts in the build-tree when changing configuration

 file(GENERATE
   OUTPUT ${POLLY_CONFIG_CMAKE_DIR}/${POLLY_EXPORTS_FILE_NAME}
   CONTENT "${POLLY_EXPORTS}")
 
 
 # Generate PollyConfig.cmake for the install tree.
 unset(POLLY_EXPORTS)
 find_prefix_from_config(POLLY_CONFIG_CODE POLLY_INSTALL_PREFIX "${POLLY_INSTALL_PACKAGE_DIR}")
-set(POLLY_CONFIG_LLVM_CMAKE_DIR "\${POLLY_INSTALL_PREFIX}/${LLVM_INSTALL_PACKAGE_DIR}")
-set(POLLY_CONFIG_CMAKE_DIR "\${POLLY_INSTALL_PREFIX}/${POLLY_INSTALL_PACKAGE_DIR}")
-set(POLLY_CONFIG_LIBRARY_DIRS "\${POLLY_INSTALL_PREFIX}/lib${LLVM_LIBDIR_SUFFIX}")
+extend_path(POLLY_CONFIG_LLVM_CMAKE_DIR "\${POLLY_INSTALL_PREFIX}" "${LLVM_INSTALL_PACKAGE_DIR}")
+extend_path(POLLY_CONFIG_CMAKE_DIR "\${POLLY_INSTALL_PREFIX}" "${POLLY_INSTALL_PACKAGE_DIR}")
+extend_path(POLLY_CONFIG_LIBRARY_DIRS "\${POLLY_INSTALL_PREFIX}" "lib${LLVM_LIBDIR_SUFFIX}")
+extend_path(base_includedir "\${POLLY_INSTALL_PREFIX}" "${CMAKE_INSTALL_INCLUDEDIR}")
 if (POLLY_BUNDLED_ISL)
   set(POLLY_CONFIG_INCLUDE_DIRS
-    "\${POLLY_INSTALL_PREFIX}/include"
-    "\${POLLY_INSTALL_PREFIX}/include/polly"
+    "${base_includedir}"
+    "${base_includedir}/polly"
     )
 else()
   set(POLLY_CONFIG_INCLUDE_DIRS
-    "\${POLLY_INSTALL_PREFIX}/include"
+    "${base_includedir}"
     ${ISL_INCLUDE_DIRS}
     )
 endif()
 
 # Set locations for imported targets. The path is constructed to be relative to
 # the config file.
 #
 # Include the config code of PollyConfig.cmake to define POLLY_INSTALL_PREFIX
 # anew in case the export file is included alone rather than from that file.
 set(POLLY_EXPORTS
 "${POLLY_CONFIG_CODE}
 
 ")
 foreach(tgt IN LISTS POLLY_CONFIG_EXPORTED_TARGETS)
   get_target_property(tgt_type ${tgt} TYPE)
   if (tgt_type STREQUAL "EXECUTABLE")
-    set(tgt_prefix "bin/")
+    set(tgt_prefix "${CMAKE_INSTALL_BINDIR}")
   else()
-    set(tgt_prefix "lib/")
+    set(tgt_prefix "${CMAKE_INSTALL_LIBDIR}")
   endif()
 
-  set(tgt_path "\${POLLY_INSTALL_PREFIX}/${tgt_prefix}$<TARGET_FILE_NAME:${tgt}>")
+  extend_path(tgt_path "\${POLLY_INSTALL_PREFIX}" "${tgt_prefix}/$<TARGET_FILE_NAME:${tgt}>")
 
   if (NOT tgt_type STREQUAL "INTERFACE_LIBRARY")
     set(POLLY_EXPORTS
 "set_target_properties(${tgt} PROPERTIES
         IMPORTED_LOCATION$<$<NOT:$<CONFIG:>>:_$<UPPER_CASE:$<CONFIG>>> \"${tgt_path}\")
 ${POLLY_EXPORTS}")
   endif()
 endforeach(tgt)

polly/lib/External/CMakeLists.txt

add_polly_library(PollyISL
     ${ISL_FILES}
     )
 
 
   if (NOT LLVM_INSTALL_TOOLCHAIN_ONLY)
     install(DIRECTORY
       ${ISL_SOURCE_DIR}/include/
       ${ISL_BINARY_DIR}/include/
-      DESTINATION include/polly
+      DESTINATION "${CMAKE_INSTALL_INCLUDEDIR}/polly"
       FILES_MATCHING
       PATTERN "*.h"
       PATTERN "CMakeFiles" EXCLUDE
       )
   endif()
 
   add_executable(polly-isl-test
     isl/isl_test.c