Avoid overwriting config.h with generated header

Change name of generated header file to config_ac.h
and use __has_include to prefer the generated file if
it is present.  Avoids stomping on the config.h used by
XCode when building using autotools.

Author: dgrove-oss

configure.ac

@@ -6,7 +6,7 @@ AC_PREREQ(2.69)
 AC_INIT([libdispatch], [1.3], [[email protected]], [libdispatch], [http://libdispatch.macosforge.org])
 AC_REVISION([$$])
 AC_CONFIG_AUX_DIR(config)
-AC_CONFIG_HEADER([config/config.h])
+AC_CONFIG_HEADER([config/config_ac.h])
 AC_CONFIG_MACRO_DIR([m4])
 ac_clean_files=a.out.dSYM
 AM_MAINTAINER_MODE

src/internal.h

@@ -27,7 +27,11 @@
 #ifndef __DISPATCH_INTERNAL__
 #define __DISPATCH_INTERNAL__
 
+#if __has_include(<config/config_ac.h>)
+#include <config/config_ac.h>
+#else
 #include <config/config.h>
+#endif
 
 #define __DISPATCH_BUILDING_DISPATCH__
 #define __DISPATCH_INDIRECT__

src/shims/linux_stubs.c

@@ -18,7 +18,12 @@
 
 #include <stdint.h>
 #include <syscall.h>
+
+#if __has_include(<config/config_ac.h>)
+#include <config/config_ac.h>
+#else
 #include <config/config.h>
+#endif
 
 #include "pthread.h"
 #include "os/linux_base.h"