cmake: Fix Android build.

Android/Bionic C library needs no special flags to have threading support.
Found when trying to build with vcpkg.

CMakeLists.txt

@@ -556,6 +556,11 @@ endif()
 if (NOT EVENT__DISABLE_THREAD_SUPPORT)
     if (WIN32)
         list(APPEND SRC_CORE evthread_win32.c)
+    elseif(ANDROID)
+        # pthreads is built in to bionic
+        set(EVENT__HAVE_PTHREADS 1)
+        CHECK_TYPE_SIZE(pthread_t EVENT__SIZEOF_PTHREAD_T)
+        list(APPEND SYMBOLS_TO_CHECK pthread_mutexattr_setprotocol)
     else()
         find_package(Threads REQUIRED)
         if (NOT CMAKE_USE_PTHREADS_INIT)