Check availability of constructor attribute and use it on Windows in …

…favor of DllMain

configure.ac

@@ -82,6 +82,24 @@ case ${host_os} in
 esac
 AM_CONDITIONAL(WIN32, test x$win32 = xtrue)
 
+# Check if the C compiler supports __attribute__((constructor))
+AC_CACHE_CHECK([wether the C compiler supports constructor/destructor attributes],
+  ac_cv_attribute_constructor, [
+  ac_cv_attribute_constructor=no
+  AC_COMPILE_IFELSE([AC_LANG_PROGRAM(
+    [[
+      static void __attribute__((constructor)) test_constructor(void) {
+      }
+      static void __attribute__((destructor)) test_destructor(void) {
+      }
+    ]], [])],
+    [ac_cv_attribute_constructor=yes]
+  )]
+)
+if test "$ac_cv_attribute_constructor" = "yes"; then
+  AC_DEFINE(HAVE_ATTRIBUTE_CONSTRUCTOR, 1, [Define if the C compiler supports constructor/destructor attributes])
+fi
+
 AC_ARG_WITH([dummy],
 	[AS_HELP_STRING([--with-dummy], [Use no USB driver at all [default=no]. This is only useful if you just want to query the device list by product type or hardware model. All other operations are no-ops or will return IRECV_E_UNSUPPORTED.])],
 	[],

src/libirecovery.c

@@ -442,7 +442,23 @@ static void _irecv_deinit(void)
 static thread_once_t init_once = THREAD_ONCE_INIT;
 static thread_once_t deinit_once = THREAD_ONCE_INIT;
 
-#ifdef WIN32
+#ifndef HAVE_ATTRIBUTE_CONSTRUCTOR
+  #if defined(__llvm__) || defined(__GNUC__)
+    #define HAVE_ATTRIBUTE_CONSTRUCTOR
+  #endif
+#endif
+
+#ifdef HAVE_ATTRIBUTE_CONSTRUCTOR
+static void __attribute__((constructor)) libirecovery_initialize(void)
+{
+	thread_once(&init_once, _irecv_init);
+}
+
+static void __attribute__((destructor)) libirecovery_deinitialize(void)
+{
+	thread_once(&deinit_once, _irecv_deinit);
+}
+#elif defined(WIN32)
 BOOL WINAPI DllMain(HINSTANCE hModule, DWORD dwReason, LPVOID lpReserved)
 {
 	switch (dwReason) {
@@ -458,15 +474,7 @@ BOOL WINAPI DllMain(HINSTANCE hModule, DWORD dwReason, LPVOID lpReserved)
 	return 1;
 }
 #else
-static void __attribute__((constructor)) libirecovery_initialize(void)
-{
-	thread_once(&init_once, _irecv_init);
-}
-
-static void __attribute__((destructor)) libirecovery_deinitialize(void)
-{
-	thread_once(&deinit_once, _irecv_deinit);
-}
+#warning No compiler support for constructor/destructor attributes, some features might not be available.
 #endif
 
 #ifdef HAVE_IOKIT