c++11: improve configure.

* configure.ac, m4/ax_check_compile_flag.m4: Update from
Vaucanson 2.  (Thank you Akim.)

configure.ac

@@ -72,26 +72,11 @@ AX_CHECK_COMPILE_FLAG([-Werror -fvisibility=hidden],
 CXXFLAGS="$CXXFLAGS -DSPOT_BUILD"
 
 # Turn on C++11 support
-AX_CHECK_COMPILE_FLAG([-std=c++11 -stdlib=libc++],
-                      [CXXFLAGS="$CXXFLAGS -std=c++11 -stdlib=libc++"
-                       stdpass=true])
-if ! "${stdpass-false}"; then
-  AX_CHECK_COMPILE_FLAG([-std=c++11],
-                        [CXXFLAGS="$CXXFLAGS -std=c++11"
-                         stdpass=true])
-fi
-if ! "${stdpass-false}"; then
-  AX_CHECK_COMPILE_FLAG([-std=c++0x],
-                        [CXXFLAGS="$CXXFLAGS -std=c++0x"
-                         stdpass=true])
-fi
-
-if ! "${stdpass-false}"; then
-  AC_ERROR([unable to turn on C++11 mode with this compiler])
-fi
+m4_define([_AX_CXX_COMPILE_STDCXX_11_testbody],
+[AC_LANG_SOURCE([#include <memory>
+#include <string>
 
-m4_define([_AX_CXX_COMPILE_STDCXX_11_testbody], [
-  template <typename T>
+   template <typename T>
     struct check
     {
       static_assert(sizeof(int) <= sizeof(T), "not big enough");
@@ -99,6 +84,8 @@ m4_define([_AX_CXX_COMPILE_STDCXX_11_testbody], [
 
     typedef check<check<bool>> right_angle_brackets;
 
+    auto f = std::make_shared<std::string>("shared_ptr");
+
     int a;
     decltype(a) b;
 
@@ -107,16 +94,17 @@ m4_define([_AX_CXX_COMPILE_STDCXX_11_testbody], [
     check_type&& cr = static_cast<check_type&&>(c);
 
     auto d = a;
-])
-
-AC_CACHE_CHECK([whether $CXX $CXXFLAGS supports C++11 features],
-               [ax_cv_cxx_compile_cxx11],
-[AC_COMPILE_IFELSE([AC_LANG_SOURCE([_AX_CXX_COMPILE_STDCXX_11_testbody])],
-  [ax_cv_cxx_compile_cxx11=yes],
-  [ax_cv_cxx_compile_cxx11=no])])
-
-test "x$ax_cv_cxx_compile_cxx11" = xyes ||
-  AC_ERROR([unable to compile C++11 with this compiler])
+])])
+
+for f in -std=c++11 '-std=c++11 -stdlib=libc++' -std=c++0x
+do
+  AX_CHECK_COMPILE_FLAG([$f], [CXXFLAGS="$CXXFLAGS $f" stdpass=true], [], [],
+                        [_AX_CXX_COMPILE_STDCXX_11_testbody])
+  ${stdpass-false} && break
+done
+if ! "${stdpass-false}"; then
+  AC_ERROR([unable to turn on C++11 mode with this compiler])
+fi
 
 AX_CHECK_BUDDY
 AX_CHECK_GSPNLIB

m4/ax_check_compile_flag.m4

@@ -61,12 +61,10 @@ AS_VAR_PUSHDEF([CACHEVAR],[ax_cv_check_[]_AC_LANG_ABBREV[]flags_$4_$1])dnl
 AC_CACHE_CHECK([whether _AC_LANG compiler accepts $1], CACHEVAR, [
   ax_check_save_flags=$[]_AC_LANG_PREFIX[]FLAGS
   _AC_LANG_PREFIX[]FLAGS="$[]_AC_LANG_PREFIX[]FLAGS $4 $1"
-  AC_COMPILE_IFELSE([AC_LANG_PROGRAM()],
+  AC_COMPILE_IFELSE([m4_default([$5], [AC_LANG_PROGRAM()])],
     [AS_VAR_SET(CACHEVAR,[yes])],
     [AS_VAR_SET(CACHEVAR,[no])])
   _AC_LANG_PREFIX[]FLAGS=$ax_check_save_flags])
-AS_IF([test x"AS_VAR_GET(CACHEVAR)" = xyes],
-  [m4_default([$2], :)],
-  [m4_default([$3], :)])
+AS_IF([test x"AS_VAR_GET(CACHEVAR)" = xyes], [$2], [$3])
 AS_VAR_POPDEF([CACHEVAR])dnl
 ])dnl AX_CHECK_COMPILE_FLAGS