Add pthread flags explicitly (#16)

Summary:
This is needed for the PyTorch host build because it includes libgtest.a
but doesn't provide any indication that it depends on pthreads.  Should
be totally safe on Android.
Pull Request resolved: #16

Test Plan: CI.  Not sure how to test the OSS Android build.

Reviewed By: passy

Differential Revision: D18056874

Pulled By: dreiss

fbshipit-source-id: 5c8044a4285f991c46cbc9665c901e0f6e9a17d0

CMakeLists.txt

@@ -9,6 +9,7 @@ project(fbjni CXX)
 set(FBJNI_COMPILE_OPTIONS
   -Wall
   -std=c++11
+  -pthread
   -fno-omit-frame-pointer
   -fexceptions
   -frtti

test/jni/CMakeLists.txt

@@ -5,6 +5,8 @@
 
 enable_testing()
 include(GoogleTest)
+set(THREADS_PREFER_PTHREAD_FLAG ON)
+find_package(Threads REQUIRED)
 
 set(TEST_COMPILE_OPTIONS
   "${FBJNI_COMPILE_OPTIONS}"
@@ -64,6 +66,7 @@ target_compile_options(modified_utf8_test PRIVATE ${TEST_COMPILE_OPTIONS})
 target_link_libraries(modified_utf8_test
   fbjni
   gtest
+  Threads::Threads
   dl
 )
 gtest_add_tests(TARGET modified_utf8_test)
@@ -75,6 +78,7 @@ target_compile_options(utf16toUTF8_test PRIVATE ${TEST_COMPILE_OPTIONS})
 target_link_libraries(utf16toUTF8_test
   fbjni
   gtest
+  Threads::Threads
   dl
 )
 gtest_add_tests(TARGET utf16toUTF8_test)