remove _GNU_SOURCE define again and add AC_FUNC_STRTOLD autoconf macro

[r27325]

configure.ac

@@ -143,17 +143,7 @@ AC_C_LONG_DOUBLE
 dnl Checks for library functions.
 dnl =============================
 
-dnl check for strerror
-
-AC_CHECK_FUNC(strerror,,
-  AC_MSG_ERROR("need strerror function")
-)
-
-dnl we use the gnu extension obstack_printf
-AC_DEFINE([_GNU_SOURCE], [], [we use gnu extensions])
-AC_CHECK_FUNC(strerror,,
-  AC_MSG_ERROR("need obstack_printf function")
-)
+AC_FUNC_STRTOLD
 
 dnl Error messaging
 dnl ===============

ir/tv/fltcalc.c

@@ -44,8 +44,7 @@
 
 #include "xmalloc.h"
 
-#if !defined(HAVE_LONG_DOUBLE) || defined(__CYGWIN__)
-/* No strtold on windows and no replacement yet */
+#ifndef HAVE_STRTOLD
 #define strtold(s, e) strtod(s, e)
 #endif