Use C++14 compiler if possible

This allows using new features of C++14 conditionally.

Signed-off-by: Stefan Weil <sw@weilnetz.de>

configure.ac

@@ -331,46 +331,42 @@ AC_COMPILE_IFELSE(
 
 AC_MSG_RESULT([$CLANG])
 
-dnl ********************
-dnl turn on c++11
-dnl ********************
+dnl **********************
+dnl Turn on C++11 or newer
+dnl **********************
 
-OLD_CXXFLAGS=$CXXFLAGS
-AC_MSG_CHECKING([whether compiler supports C++11])
-CXXFLAGS="$CXXFLAGS -std=c++11"
-AC_COMPILE_IFELSE(
-[
-  AC_LANG_SOURCE([[
-    #if (__cplusplus < 201103L)
-    #error C++ 11 is unsupported
-    #endif
-  ]])
-], [
-     AC_MSG_RESULT(yes)
-   ],
-   [
-     AC_MSG_RESULT(no)
-     AC_MSG_ERROR([Your compiler does not have the necessary c++11 support! Cannot proceed.])
-   ])
-CXXFLAGS="$OLD_CXXFLAGS"
-
-
-# set c++11 support based on platform/compiler
+CPLUSPLUS=
+
+AX_CHECK_COMPILE_FLAG([-std=c++11], [cplusplus11=true], [cplusplus11=false])
+if $cplusplus11; then
+  CPLUSPLUS=11
+fi
+
+AX_CHECK_COMPILE_FLAG([-std=c++14], [cplusplus14=true], [cplusplus14=false])
+if $cplusplus14; then
+  CPLUSPLUS=14
+fi
+
+if test -z "$CPLUSPLUS"; then
+  AC_MSG_ERROR([Your compiler does not have the necessary C++11 support! Cannot proceed.])
+fi
+
+# Set C++11 or C++14 support based on platform/compiler
 case "${host_os}" in
   cygwin*)
-    CXXFLAGS="$CXXFLAGS -std=gnu++11"
+    CXXFLAGS="$CXXFLAGS -std=gnu++$CPLUSPLUS"
     ;;
   *-darwin* | *-macos10*)
      if test "x$CLANG" = "xyes"; then
-       CXXFLAGS="$CXXFLAGS -std=c++11 "
+       CXXFLAGS="$CXXFLAGS -std=c++$CPLUSPLUS"
        LDFLAGS="$LDFLAGS -stdlib=libc++"
      else
-       CXXFLAGS="$CXXFLAGS -std=c++11"
+       CXXFLAGS="$CXXFLAGS -std=c++$CPLUSPLUS"
      fi
     ;;
   *)
     # default
-    CXXFLAGS="$CXXFLAGS -std=c++11"
+    CXXFLAGS="$CXXFLAGS -std=c++$CPLUSPLUS"
     ;;
 esac