CMakeLists.txt: fix install on macOS if GNU coreutils are used (#203)

* CMakeLists.txt: fix install on macOS if GNU coreutils are used

bsd ln supports both `-h` and `-n`, while GNU only supports `-n`.
In Nix we use GNU coreutils and hence the installation breaks.

Also see NixOS/nixpkgs#182909

With this patch installation will work with both variants of `ln`

* CMakeLists.txt: use CMAKE_INSTALL_FULL_INCLUDEDIR for header files

This allows to install header files in prefixes different from the rest.
We need this for nix as well.

CMakeLists.txt

@@ -22,18 +22,18 @@ add_library(OpenCL::Headers ALIAS Headers)
 
 if(CMAKE_SYSTEM_NAME MATCHES Darwin)
   execute_process(
-    COMMAND ln -shf ${CMAKE_CURRENT_SOURCE_DIR}/CL ${CMAKE_CURRENT_BINARY_DIR}/CL
-    COMMAND ln -shf ${CMAKE_CURRENT_SOURCE_DIR}/CL ${CMAKE_CURRENT_BINARY_DIR}/OpenCL
+    COMMAND ln -snf ${CMAKE_CURRENT_SOURCE_DIR}/CL ${CMAKE_CURRENT_BINARY_DIR}/CL
+    COMMAND ln -snf ${CMAKE_CURRENT_SOURCE_DIR}/CL ${CMAKE_CURRENT_BINARY_DIR}/OpenCL
   )
   target_include_directories(Headers
     INTERFACE
       $<BUILD_INTERFACE:${CMAKE_CURRENT_BINARY_DIR}>
       $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>
   )
   install(CODE "
-    file(MAKE_DIRECTORY  ${CMAKE_INSTALL_PREFIX}/${CMAKE_INSTALL_INCLUDEDIR}/CL)
+    file(MAKE_DIRECTORY  ${CMAKE_INSTALL_FULL_INCLUDEDIR}/CL)
     execute_process(
-      COMMAND ln -shf CL ${CMAKE_INSTALL_PREFIX}/${CMAKE_INSTALL_INCLUDEDIR}/OpenCL
+      COMMAND ln -snf CL ${CMAKE_INSTALL_FULL_INCLUDEDIR}/OpenCL
     )"
   )
 else()