Fix missing C declarations causing compiler warnings

This fixes declarations for ruby-2.0 and newer.
rubies < 2.0 don't define ruby/thread.h, so that the warnings are still present there.

compiling ../../../../ext/ffi_c/Call.c
In file included from ../../../../ext/ffi_c/Call.h:36:0,
                 from ../../../../ext/ffi_c/Function.h:48,
                 from ../../../../ext/ffi_c/Call.c:57:
../../../../ext/ffi_c/Call.c: In function ‘rbffi_do_blocking_call’:
../../../../ext/ffi_c/Thread.h:78:39: warning: implicit declaration of function ‘rb_thread_call_without_gvl’; did you mean ‘rb_thread_wait_for’? [-Wimplicit-function-declaration]
 # define rbffi_thread_blocking_region rb_thread_call_without_gvl
                                       ^
../../../../ext/ffi_c/Call.c:334:5: note: in expansion of macro ‘rbffi_thread_blocking_region’
     rbffi_thread_blocking_region(call_blocking_function, data, (void *) -1, NULL);
     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
compiling ../../../../ext/ffi_c/Function.c
../../../../ext/ffi_c/Function.c: In function ‘async_cb_event’:
../../../../ext/ffi_c/Function.c:569:9: warning: implicit declaration of function ‘rb_thread_call_without_gvl’; did you mean ‘rb_thread_call_with_gvl’? [-Wimplicit-function-declaration]
         rb_thread_call_without_gvl(async_cb_wait, &w, async_cb_stop, &w);
         ^~~~~~~~~~~~~~~~~~~~~~~~~~
         rb_thread_call_with_gvl

ext/ffi_c/Call.c

@@ -43,6 +43,9 @@
 #endif
 #include <errno.h>
 #include <ruby.h>
+#if defined(HAVE_RUBY_THREAD_H)
+#include <ruby/thread.h>
+#endif
 #if defined(HAVE_NATIVETHREAD) && (defined(HAVE_RB_THREAD_BLOCKING_REGION) || defined(HAVE_RB_THREAD_CALL_WITHOUT_GVL)) && !defined(_WIN32)
 #  include <signal.h>
 #  include <pthread.h>
@@ -317,15 +320,15 @@ rbffi_SetupCallParams(int argc, VALUE* argv, int paramCount, Type** paramTypes,
     }
 }
 
-static VALUE
+static void *
 call_blocking_function(void* data)
 {
     rbffi_blocking_call_t* b = (rbffi_blocking_call_t *) data;
     b->frame->has_gvl = false;
     ffi_call(&b->cif, FFI_FN(b->function), b->retval, b->ffiValues);
     b->frame->has_gvl = true;
 
-    return Qnil;
+    return NULL;
 }
 
 VALUE

ext/ffi_c/Function.c

@@ -47,6 +47,9 @@
 # endif
 #endif
 #include <ruby.h>
+#if defined(HAVE_RUBY_THREAD_H)
+#include <ruby/thread.h>
+#endif
 
 #include <ffi.h>
 #if defined(HAVE_NATIVETHREAD) && !defined(_WIN32)
@@ -554,7 +557,7 @@ struct async_wait {
     bool stop;
 };
 
-static VALUE async_cb_wait(void *);
+static void * async_cb_wait(void *);
 static void async_cb_stop(void *);
 
 #if defined(HAVE_RB_THREAD_BLOCKING_REGION) || defined(HAVE_RB_THREAD_CALL_WITHOUT_GVL)
@@ -643,7 +646,7 @@ async_cb_event(void* unused)
 #endif
 
 #ifdef _WIN32
-static VALUE
+static void *
 async_cb_wait(void *data)
 {
     struct async_wait* w = (struct async_wait *) data;
@@ -665,7 +668,7 @@ async_cb_wait(void *data)
 
     LeaveCriticalSection(&async_cb_lock);
 
-    return Qnil;
+    return NULL;
 }
 
 static void
@@ -680,7 +683,7 @@ async_cb_stop(void *data)
 }
 
 #else
-static VALUE
+static void *
 async_cb_wait(void *data)
 {
     struct async_wait* w = (struct async_wait *) data;
@@ -700,7 +703,7 @@ async_cb_wait(void *data)
 
     pthread_mutex_unlock(&async_cb_mutex);
 
-    return Qnil;
+    return NULL;
 }
 
 static void

ext/ffi_c/extconf.rb

@@ -26,6 +26,7 @@
   end
 
   have_header('shlwapi.h')
+  have_header('ruby/thread.h') # for compat with ruby < 2.0
   have_func('rb_thread_blocking_region')
   have_func('rb_thread_call_with_gvl')
   have_func('rb_thread_call_without_gvl')