configure: do not redefine _REENTRANT

This fixes warnings with PulseAudio and with MingW.

configure.ac

@@ -56,7 +56,11 @@ AC_PROG_CC_C99
 AC_USE_SYSTEM_EXTENSIONS
 AC_DEFINE([_FORTIFY_SOURCE], 2, [Define to 2 to get glibc warnings.])
 AC_DEFINE([_FILE_OFFSET_BITS], 64, [Define to 64 for large files support.])
-AC_DEFINE([_REENTRANT],, [Define to expose reentrant functions.])
+AH_TOP([
+#ifndef _REENTRANT
+# define _REENTRANT
+#endif
+])
 AC_DEFINE([_THREAD_SAFE],, [Same as _REENTANT for some other OSes.])
 AC_DEFINE([__LIBVLC__],, [Define within the LibVLC source code tree.])
 AC_DEFINE([WIN32_LEAN_AND_MEAN],, [Define to limit the scope of <windows.h>.])