remove pthreadpool.a from install directory (pytorch#25977)

Summary:
Pull Request resolved: pytorch#25977

Call add_subdirectory() explicitly before NNPACK/QNNPACK with
EXCLUDE_FROM_ALL property so that pthreadpool target won't be installed
by default for libtorch mobile build.

Pull Request resolved: pytorch#25977

Test Plan: Imported from OSS

Differential Revision: D17312083

Pulled By: ljk53

fbshipit-source-id: 79851d0aa9402c5b9287ef4bbd8d7fd3a341497d

cmake/Dependencies.cmake

@@ -186,6 +186,28 @@ set(CONFU_DEPENDENCIES_SOURCE_DIR ${PROJECT_BINARY_DIR}/confu-srcs
 set(CONFU_DEPENDENCIES_BINARY_DIR ${PROJECT_BINARY_DIR}/confu-deps
   CACHE PATH "Confu-style dependencies binary directory")
 
+# ---[ pthreadpool
+# QNNPACK and NNPACK both depend on pthreadpool, but when building with libtorch
+# they should use the pthreadpool implementation under caffe2/utils/threadpool
+# instead of the default implementation. To avoid confusion, add pthreadpool
+# subdirectory explicitly with EXCLUDE_FROM_ALL property prior to QNNPACK/NNPACK
+# does so, which will prevent it from installing the default pthreadpool library.
+if(INTERN_BUILD_MOBILE AND NOT BUILD_CAFFE2_MOBILE AND (USE_QNNPACK OR USE_NNPACK))
+  if(NOT DEFINED PTHREADPOOL_SOURCE_DIR)
+    set(CAFFE2_THIRD_PARTY_ROOT "${PROJECT_SOURCE_DIR}/third_party")
+    set(PTHREADPOOL_SOURCE_DIR "${CAFFE2_THIRD_PARTY_ROOT}/pthreadpool" CACHE STRING "pthreadpool source directory")
+  endif()
+
+  IF(NOT TARGET pthreadpool)
+    SET(PTHREADPOOL_BUILD_TESTS OFF CACHE BOOL "")
+    SET(PTHREADPOOL_BUILD_BENCHMARKS OFF CACHE BOOL "")
+    ADD_SUBDIRECTORY(
+      "${PTHREADPOOL_SOURCE_DIR}"
+      "${CONFU_DEPENDENCIES_BINARY_DIR}/pthreadpool"
+      EXCLUDE_FROM_ALL)
+  ENDIF()
+endif()
+
 # ---[ QNNPACK
 if(USE_QNNPACK)
   if (IOS)