Disable checks for C++11 if C++0x is disabled

By adding an extra test we can avoid the somehow illogical possibility
of building with '--enable-gxx11check --disable-gxx0xcheck'.

m4/cxx0x_compiler.m4

@@ -4,15 +4,22 @@
 AC_DEFUN([GXX0X],[
   ac_save_CXX="$CXX"
 
-  # try flag -std=c++11
-  AC_CACHE_CHECK([whether $CXX accepts -std=c++11], dune_cv_gplusplus_accepts_cplusplus11, [
-    AC_REQUIRE([AC_PROG_CXX])
+  # put this check first, so we get disable C++11 if C++0x is
+  AC_ARG_ENABLE(gxx0xcheck,
+    AC_HELP_STRING([--disable-gxx0xcheck],
+      [try flag -std=c++0x to enable C++0x features [[default=yes]]]),
+      [gxx0xcheck=$enableval],
+      [gxx0xcheck=yes])
   AC_ARG_ENABLE(gxx11check,
     AC_HELP_STRING([--disable-gxx11check],
       [try flag -std=c++11 to enable C++11 features [[default=yes]]]),
       [gxx11check=$enableval],
       [gxx11check=yes])
-    if test "x$GXX" = xyes && test "x$gxx11check" = xyes; then
+
+  # try flag -std=c++11
+  AC_CACHE_CHECK([whether $CXX accepts -std=c++11], dune_cv_gplusplus_accepts_cplusplus11, [
+    AC_REQUIRE([AC_PROG_CXX])
+    if test "x$GXX" = xyes && test "x$gxx11check" = xyes && test "x$gxx0xcheck" = xyes ; then
       AC_LANG_PUSH([C++])
       CXX="$CXX -std=c++11"
       AC_TRY_COMPILE([
@@ -22,6 +29,8 @@ AC_DEFUN([GXX0X],[
         dune_cv_gplusplus_accepts_cplusplus11=yes,
         dune_cv_gplusplus_accepts_cplusplus11=no)
       AC_LANG_POP([C++])
+    else
+      dune_cv_gplusplus_accepts_cplusplus11=disabled
     fi
   ])
   if test "x$dune_cv_gplusplus_accepts_cplusplus11" == "xyes" ; then
@@ -33,11 +42,6 @@ AC_DEFUN([GXX0X],[
     # try flag -std=c++0x instead
     AC_CACHE_CHECK([whether $CXX accepts -std=c++0x], dune_cv_gplusplus_accepts_cplusplus0x, [
       AC_REQUIRE([AC_PROG_CXX])
-      AC_ARG_ENABLE(gxx0xcheck,
-        AC_HELP_STRING([--disable-gxx0xcheck],
-          [try flag -std=c++0x to enable C++11 features [[default=yes]]]),
-          [gxx0xcheck=$enableval],
-          [gxx0xcheck=yes])
       if test "x$GXX" = xyes && test "x$gxx0xcheck" = xyes; then
         AC_LANG_PUSH([C++])
         CXX="$CXX -std=c++0x"
@@ -48,6 +52,8 @@ AC_DEFUN([GXX0X],[
           dune_cv_gplusplus_accepts_cplusplus0x=yes,
           dune_cv_gplusplus_accepts_cplusplus0x=no)
         AC_LANG_POP([C++])
+      else
+        dune_cv_gplusplus_accepts_cplusplus0x=disabled
       fi
     ])
     if test "x$dune_cv_gplusplus_accepts_cplusplus0x" == "xyes" ; then