detect llvm toolchain utilities #392

Use `clang -print-prog-path=<tool>` to find the locations of llvm
toolchain utilities such as `llvm-ranlib` and set the appropriate cmake
variables to the resultant paths.

Signed-off-by: Rafael Kitover <rkitover@gmail.com>

CMakeLists.txt

@@ -359,7 +359,11 @@ endif()
 include(ProcessorCount)
 ProcessorCount(num_cpus)
 
-# Compiler flags
+# Compiler stuff
+
+if(CMAKE_C_COMPILER_ID STREQUAL Clang AND CMAKE_CXX_COMPILER_ID STREQUAL Clang)
+    include(LLVMToolchain)
+endif()
 
 if(CMAKE_COMPILER_IS_GNUCXX OR CMAKE_CXX_COMPILER_ID STREQUAL Clang)
     set(LTO_FLAGS "")

cmake/LLVMToolchain.cmake

@@ -0,0 +1,32 @@
+function(use_llvm_toolchain)
+    if(CMAKE_C_COMPILER_ID STREQUAL Clang)
+        set(compiler "${CMAKE_C_COMPILER}")
+    elseif(CMAKE_CXX_COMPILER_ID STREQUAL Clang)
+        set(compiler "${CMAKE_CXX_COMPILER}")
+    else()
+        return()
+    endif()
+
+    foreach(tool ar ranlib ld nm objdump as)
+        execute_process(
+            COMMAND "${compiler}" -print-prog-name=llvm-${tool}
+            OUTPUT_VARIABLE prog_path
+            OUTPUT_STRIP_TRAILING_WHITESPACE
+        )
+
+        if(prog_path MATCHES "^/")
+            if(tool STREQUAL ld)
+                set(tool linker)
+            elseif(tool STREQUAL as)
+                set(tool asm_compiler)
+            endif()
+
+            string(TOUPPER ${tool} utool)
+
+            set(CMAKE_${utool} "${prog_path}" PARENT_SCOPE)
+            set(CMAKE_${utool} "${prog_path}" CACHE FILEPATH "${tool}" FORCE)
+        endif()
+    endforeach()
+endfunction()
+
+use_llvm_toolchain()