CMake/deps: Fix LLVM and OSL builds

This commit is addition to D10212. Apple arm64 support was left out in that patch.

build_files/build_environment/cmake/ispc.cmake

@@ -24,11 +24,12 @@ if(WIN32)
     -DARM_ENABLED=Off
   )
 elseif(APPLE)
-  # Use bison installed via Homebrew.
-  # The one which comes which Xcode toolset is too old.
+  # Use bison and flex installed via Homebrew.
+  # The ones that come with Xcode toolset are too old.
   if("${CMAKE_HOST_SYSTEM_PROCESSOR}" STREQUAL "arm64")
     set(ISPC_EXTRA_ARGS_APPLE
       -DBISON_EXECUTABLE=/opt/homebrew/opt/bison/bin/bison
+      -DFLEX_EXECUTABLE=/opt/homebrew/opt/flex/bin/flex
       -DARM_ENABLED=On
     )
   else()

build_files/build_environment/cmake/osl.cmake

@@ -71,6 +71,11 @@ set(OSL_EXTRA_ARGS
   -DUSE_PYTHON=OFF
 )
 
+# Apple arm64 uses LLVM 11, LLVM 10+ requires C++14
+if (APPLE AND "${CMAKE_OSX_ARCHITECTURES}" STREQUAL "arm64")
+  list(APPEND OSL_EXTRA_ARGS -DCMAKE_CXX_STANDARD=14)
+endif()
+
 ExternalProject_Add(external_osl
   URL ${OSL_URI}
   DOWNLOAD_DIR ${DOWNLOAD_DIR}

build_files/build_environment/cmake/versions.cmake

@@ -116,8 +116,8 @@ set(OPENCOLORIO_HASH 1a2e3478b6cd9a1549f24e1b2205e3f0)
 if(APPLE AND ("${CMAKE_OSX_ARCHITECTURES}" STREQUAL "arm64"))
   # Newer version required by ISPC with arm support.
   set(LLVM_VERSION 11.0.1)
-  set(LLVM_URI https://github.com/llvm/llvm-project/releases/download/llvmorg-${LLVM_VERSION}/llvm-project-${LLVM_VERSION}.tar.xz)
-  set(LLVM_HASH 6ec7ae9fd43da9b87cda15b3ab9cc7af)
+  set(LLVM_URI https://github.com/llvm/llvm-project/releases/download/llvmorg-${LLVM_VERSION}/llvm-project-${LLVM_VERSION}.src.tar.xz)
+  set(LLVM_HASH e700af40ab83463e4e9ab0ba3708312e)
 
   set(OPENMP_VERSION 9.0.1)
   set(OPENMP_URI https://github.com/llvm/llvm-project/releases/download/llvmorg-${OPENMP_VERSION}/openmp-${OPENMP_VERSION}.src.tar.xz)