Use cmake_path(APPEND) to determine include directory

The previous code concatenates includedir to _harfbuzz_prefix verbatim, which
results in a wrong resulting include path in case includedir is an absolute
path. The right behavior would be in this case to drop _harfbuzz_prefix and
just use the absolute path which this implements using cmake_path(APPEND).

This is an issue in nixpkgs, where includedir is set to the final (absolute)
path of the built library in the Nix store, which causes CMake projects
depending on harfbuzz to not configure.

See NixOS/nixpkgs#180054.

NB: cmake_path is a function introduced in CMake 3.20, while harfbuzz's
CMakeLists.txt currently specifies 3.12.

src/harfbuzz-config.cmake.in

@@ -24,6 +24,7 @@ unset(_harfbuzz_libdir_iter)
 # Get the include subdir.
 set(_harfbuzz_includedir "@includedir@")
 string(REPLACE "${_harfbuzz_remove_string}/" "" _harfbuzz_includedir "${_harfbuzz_includedir}")
+cmake_path(APPEND "${_harfbuzz_prefix}" "${_harfbuzz_includedir}" OUTPUT_VARIABLE _harfbuzz_includedir)
 
 # Extract version information from libtool.
 set(_harfbuzz_version_info "@HB_LIBTOOL_VERSION_INFO@")
@@ -48,18 +49,18 @@ endif ()
 # Add the libraries.
 add_library(harfbuzz::harfbuzz SHARED IMPORTED)
 set_target_properties(harfbuzz::harfbuzz PROPERTIES
-  INTERFACE_INCLUDE_DIRECTORIES "${_harfbuzz_prefix}/${_harfbuzz_includedir}/harfbuzz"
+  INTERFACE_INCLUDE_DIRECTORIES "${_harfbuzz_includedir}/harfbuzz"
   IMPORTED_LOCATION "${_harfbuzz_prefix}/${_harfbuzz_libdir}/libharfbuzz${_harfbuzz_lib_suffix}")
 
 add_library(harfbuzz::icu SHARED IMPORTED)
 set_target_properties(harfbuzz::icu PROPERTIES
-  INTERFACE_INCLUDE_DIRECTORIES "${_harfbuzz_prefix}/${_harfbuzz_includedir}/harfbuzz"
+  INTERFACE_INCLUDE_DIRECTORIES "${_harfbuzz_includedir}/harfbuzz"
   INTERFACE_LINK_LIBRARIES "harfbuzz::harfbuzz"
   IMPORTED_LOCATION "${_harfbuzz_prefix}/${_harfbuzz_libdir}/libharfbuzz-icu${_harfbuzz_lib_suffix}")
 
 add_library(harfbuzz::subset SHARED IMPORTED)
 set_target_properties(harfbuzz::subset PROPERTIES
-  INTERFACE_INCLUDE_DIRECTORIES "${_harfbuzz_prefix}/${_harfbuzz_includedir}/harfbuzz"
+  INTERFACE_INCLUDE_DIRECTORIES "${_harfbuzz_includedir}/harfbuzz"
   INTERFACE_LINK_LIBRARIES "harfbuzz::harfbuzz"
   IMPORTED_LOCATION "${_harfbuzz_prefix}/${_harfbuzz_libdir}/libharfbuzz-subset${_harfbuzz_lib_suffix}")
 
@@ -68,7 +69,7 @@ set(_harfbuzz_have_gobject "@have_gobject@")
 if (_harfbuzz_have_gobject)
   add_library(harfbuzz::gobject SHARED IMPORTED)
   set_target_properties(harfbuzz::gobject PROPERTIES
-    INTERFACE_INCLUDE_DIRECTORIES "${_harfbuzz_prefix}/${_harfbuzz_includedir}/harfbuzz"
+    INTERFACE_INCLUDE_DIRECTORIES "${_harfbuzz_includedir}/harfbuzz"
     INTERFACE_LINK_LIBRARIES "harfbuzz::harfbuzz"
     IMPORTED_LOCATION "${_harfbuzz_prefix}/${_harfbuzz_libdir}/libharfbuzz-gobject${_harfbuzz_lib_suffix}")
 endif ()