Install executor_api.h when generated in OSS build (#1010)

Summary:
futures.h includes folly/python/executor_api.h, this header needs to be
available to downstream projects (Thrift) which use python/futures.h.
Pull Request resolved: #1010

Reviewed By: Orvid

Differential Revision: D13915880

Pulled By: calebmarchent

fbshipit-source-id: 3c47fc4fe4ba425a4c12dcefd8980703765bdad5

folly/CMakeLists.txt

@@ -50,9 +50,16 @@ if (PYTHON_EXTENSIONS)
     COMMAND
       python3 ${CMAKE_CURRENT_SOURCE_DIR}/setup.py
       build_ext -f ${incs} ${libs}
+    BYPRODUCTS ${_cybld}/folly/executor_api.h
     DEPENDS folly_pic
     WORKING_DIRECTORY ${_cybld})
 
+  install(
+    FILES ${_cybld}/folly/executor_api.h
+    DESTINATION ${INCLUDE_INSTALL_DIR}/folly/python
+    COMPONENT dev
+  )
+
   # Install Folly Python Bindings
   install(CODE "
     string(REGEX REPLACE \"^(..*)$\" \"--root=\\\\1\" PYROOT \"\$ENV{DESTDIR}\")