llvm: cmake: llvm_config: support INTERFACE_LIBRARY targets

[acolin]

Summary

Currently, llvm_config(target) method cannot be used with a target of type INTERFACE_LIBRARY like so:

add_library(Halide_LLVM INTERFACE)
set(LLVM_USE_SHARED "USE_SHARED")
llvm_config(Halide_LLVM ${LLVM_USE_SHARED} ${LLVM_COMPONENTS})

CMake throws an error:

    -- Using LLVMConfig.cmake in: /usr/lib/llvm/10/lib64/cmake/llvm
CMake Error at /usr/lib/llvm/10/lib64/cmake/llvm/LLVM-Config.cmake:91 (target_link_libraries):
  INTERFACE library can only be used with the INTERFACE keyword of
  target_link_libraries
Call Stack (most recent call first):
  dependencies/llvm/CMakeLists.txt:135 (llvm_config)

llvm_config() method seems to be the preferred way to link a library/app against libLLVM.so, as opposed to hardcoding the LLVM library name into a manual call to target_link_libraries(). But, can't use llvm_config() if the target happens to be of type INTERFACE_LIBRARY instead of SHARED_LIBRARY, for example.

This patch adds support for INTERFACE_LIBRARY target type to llvm_config().

This would be useful to Halide's cmake script (https://halide-lang.org).

llvm/cmake/modules/LLVM-Config.cmake

       endif()
     endif()
 
-    target_link_libraries(${executable} PRIVATE LLVM)
+    get_target_property(t ${executable} TYPE)
+    if(t STREQUAL "INTERFACE_LIBRARY")
+      set(link_mode "INTERFACE")
+    else()
+      set(link_mode "PRIVATE")
+    endif()
+    target_link_libraries(${executable} ${link_mode} LLVM)
   endif()
 
   explicit_llvm_config(${executable} ${link_components})

 
   llvm_map_components_to_libnames(LIBRARIES ${link_components})
   get_target_property(t ${executable} TYPE)
-  if(t STREQUAL "STATIC_LIBRARY")
+  if(t STREQUAL "STATIC_LIBRARY" OR t STREQUAL "INTERFACE_LIBRARY")
     target_link_libraries(${executable} INTERFACE ${LIBRARIES})
   elseif(t STREQUAL "EXECUTABLE" OR t STREQUAL "SHARED_LIBRARY" OR t STREQUAL "MODULE_LIBRARY")
     target_link_libraries(${executable} PRIVATE ${LIBRARIES})