Add CMake option to prefer static deps rather than dynamic deps

Summary:
We would like to build a version of torchcomms that has minimal dependencies on dynamic libraries. I saw that there is an option `USE_STATIC_DEPS_ON_UNIX` introduced in D17228181. However, currently when we enable `USE_STATIC_DEPS_ON_UNIX=ON` to build folly, it will fail by
```
-- Could NOT find LIBUNWIND (missing: LIBUNWIND_LIBRARY)
CMake Error at build/fbcode_builder/CMake/FindLibEvent.cmake:68 (message):
  Could NOT find libevent.
Call Stack (most recent call first):
  CMake/folly-deps.cmake:73 (find_package)
  CMakeLists.txt:145 (include)

-- Configuring incomplete, errors occurred!
```

That is because we only set ".a" to `CMAKE_FIND_LIBRARY_SUFFIXES` and there exists a special library libunwind that does not provide static linking library.

To set `CMAKE_FIND_LIBRARY_SUFFIXES = ".a" ".so"`, we can let cmake first look for `libname.a` and then `libname.so` if `.a` is not found.

Reviewed By: d4l3k

Differential Revision: D81062853

fbshipit-source-id: ce901b03473c82032c75742302ee3ed875d6af49

Author: tanquer

build/fbcode_builder/CMake/FBBuildOptions.cmake

@@ -12,4 +12,16 @@ function (fb_activate_static_library_option)
   if(UNIX AND USE_STATIC_DEPS_ON_UNIX)
     SET(CMAKE_FIND_LIBRARY_SUFFIXES ".a" PARENT_SCOPE)
   endif()
+
+  option(PREFER_STATIC_DEPS_ON_UNIX
+    "If enabled, use static dependencies on unix systems as possible as we can. This is generally discouraged."
+    OFF
+  )
+  # Mark PREFER_STATIC_DEPS_ON_UNIX as an "advanced" option, since enabling it
+  # is generally discouraged.
+  mark_as_advanced(PREFER_STATIC_DEPS_ON_UNIX)
+
+  if(UNIX AND PREFER_STATIC_DEPS_ON_UNIX)
+    SET(CMAKE_FIND_LIBRARY_SUFFIXES ".a" ".so" PARENT_SCOPE)
+  endif()
 endfunction()