deps: fix llvm using system python

llvm was using system python, rather than our copy
this went unnoticed on both linux and windows until
sergey tried to build the deps on a clean system with
no system python installed.

build_files/build_environment/CMakeLists.txt

@@ -57,7 +57,6 @@ include(cmake/alembic.cmake)
 include(cmake/opensubdiv.cmake)
 include(cmake/sdl.cmake)
 include(cmake/opencollada.cmake)
-include(cmake/llvm.cmake)
 if(APPLE)
   include(cmake/openmp.cmake)
 endif()
@@ -75,6 +74,7 @@ include(cmake/osl.cmake)
 include(cmake/tbb.cmake)
 include(cmake/openvdb.cmake)
 include(cmake/python.cmake)
+include(cmake/llvm.cmake)
 option(USE_PIP_NUMPY "Install NumPy using pip wheel instead of building from source" OFF)
 if(APPLE AND ("${CMAKE_OSX_ARCHITECTURES}" STREQUAL "x86_64"))
   set(USE_PIP_NUMPY ON)

build_files/build_environment/cmake/llvm.cmake

@@ -25,11 +25,14 @@ set(LLVM_EXTRA_ARGS
   -DLLVM_BUILD_LLVM_C_DYLIB=OFF
   -DLLVM_ENABLE_UNWIND_TABLES=OFF
   -DLLVM_ENABLE_PROJECTS=clang${LLVM_BUILD_CLANG_TOOLS_EXTRA}
+  -DPython3_ROOT_DIR=${LIBDIR}/python/
+  -DPython3_EXECUTABLE=${PYTHON_BINARY}
   ${LLVM_XML2_ARGS}
 )
 
 if(WIN32)
   set(LLVM_GENERATOR "Ninja")
+  list(APPEND LLVM_EXTRA_ARGS -DPython3_FIND_REGISTRY=NEVER)
 else()
   set(LLVM_GENERATOR "Unix Makefiles")
 endif()
@@ -74,3 +77,9 @@ if(APPLE)
     external_xml2
   )
 endif()
+
+add_dependencies(
+  ll
+  external_python
+)
+