append to CMAKE_FIND_ROOT_PATH instead of setting executorch_DIR (#12618)

I think the <package>_DIR mechanism is too low-level -- it helps find
<package>, but not any dependencies that <package> also installed next
to itself. CMAKE_PREFIX_PATH does both.

Author: swolchok

examples/models/llama/CMakeLists.txt

@@ -77,9 +77,11 @@ find_package(gflags REQUIRED)
 # llama_main: test binary to run llama, with tokenizer and sampler integrated
 #
 
-# find `executorch` libraries Same as for gflags
-set(executorch_DIR ${CMAKE_CURRENT_BINARY_DIR}/../../../lib/cmake/ExecuTorch)
-find_package(executorch CONFIG REQUIRED)
+# find `executorch` libraries. CMAKE_PREFIX_PATH would work for host
+# compilation, but CMAKE_FIND_ROOT_PATH appears to be necessary for
+# cross-compiling (e.g., to Android) to work as well.
+list(APPEND CMAKE_FIND_ROOT_PATH ${CMAKE_CURRENT_BINARY_DIR}/../../..)
+find_package(executorch CONFIG REQUIRED FIND_ROOT_PATH_BOTH)
 target_link_options_shared_lib(executorch)
 
 # llama_runner library

examples/models/llava/CMakeLists.txt

@@ -76,8 +76,8 @@ find_package(gflags REQUIRED)
 #
 
 # find `executorch` libraries Same as for gflags
-set(executorch_DIR ${CMAKE_CURRENT_BINARY_DIR}/../../../lib/cmake/ExecuTorch)
-find_package(executorch CONFIG REQUIRED)
+list(APPEND CMAKE_FIND_ROOT_PATH ${CMAKE_CURRENT_BINARY_DIR}/../../..)
+find_package(executorch CONFIG REQUIRED FIND_ROOT_PATH_BOTH)
 target_link_options_shared_lib(executorch)
 
 # llava_runner library

examples/models/phi-3-mini/CMakeLists.txt

@@ -24,8 +24,8 @@ set(EXECUTORCH_ROOT "${CMAKE_CURRENT_SOURCE_DIR}/../../..")
 set(_common_include_directories
     ${EXECUTORCH_ROOT}/.. ${EXECUTORCH_ROOT}/runtime/core/portable_type/c10
 )
-set(executorch_DIR ${CMAKE_CURRENT_BINARY_DIR}/../../../lib/cmake/ExecuTorch)
-find_package(executorch CONFIG REQUIRED)
+list(APPEND CMAKE_FIND_ROOT_PATH ${CMAKE_CURRENT_BINARY_DIR}/../../..)
+find_package(executorch CONFIG REQUIRED FIND_ROOT_PATH_BOTH)
 
 target_link_options_shared_lib(executorch)
 

extension/android/CMakeLists.txt

@@ -60,7 +60,6 @@ set_target_properties(fbjni PROPERTIES
   IMPORTED_LOCATION "${CMAKE_CURRENT_BINARY_DIR}/third-party/fbjni/prefab/modules/fbjni/libs/android.${ANDROID_ABI}/libfbjni.so"
 )
 
-set(executorch_DIR ${CMAKE_CURRENT_BINARY_DIR}/../../lib/cmake/ExecuTorch)
 target_link_options_shared_lib(executorch)
 
 add_library(executorch_jni SHARED jni/jni_layer.cpp jni/log.cpp jni/jni_layer_runtime.cpp)