Check that ffi-compiler loads before using it to define tasks. (#14538)

Fixes #14509

Closes #14538

COPYBARA_INTEGRATE_REVIEW=https://github.com/protocolbuffers/protobuf/pull/14538 from JasonLunn:fix_14509 a3f9d0bd4f
PiperOrigin-RevId: 577837541

ruby/lib/google/tasks/ffi.rake

@@ -1,5 +1,3 @@
-require "ffi-compiler/compile_task"
-
 # # @param task [FFI::Compiler::CompileTask] task to configure
 def configure_common_compile_task(task)
   if FileUtils.pwd.include? 'ext'
@@ -49,6 +47,9 @@ def with_generated_files
   end
 end
 
+begin
+  require "ffi-compiler/compile_task"
+
   desc "Compile Protobuf library for FFI"
   namespace "ffi-protobuf" do
     with_generated_files do
@@ -91,4 +92,11 @@ namespace "ffi-protobuf" do
       end
     end
   end
-
+rescue LoadError
+  desc "Compile Protobuf library for FFI"
+  namespace "ffi-protobuf" do
+    task :default do
+      warn "Skipping build of FFI; `gem install ffi-compiler` to enable."
+    end
+  end
+end