Officially require C++11.

As it turns out, support for building with a c++98
compiler was broken several months (years?) ago, but nobody
noticed so far.

This commit updates the README and header file to
officially document that situation.

README.md

@@ -65,7 +65,7 @@ int main()
 
 ![Basic example](./examples/basic.png)
 
-matplotlib-cpp doesn't require C++11, but will enable some additional syntactic sugar when available:
+Alternatively, matplotlib-cpp also supports some C++11-powered syntactic sugar:
 ```cpp
 #include <cmath>
 #include "matplotlibcpp.h"
@@ -186,6 +186,16 @@ find_package(PythonLibs 2.7)
 target_include_directories(myproject PRIVATE ${PYTHON_INCLUDE_DIRS})
 target_link_libraries(myproject ${PYTHON_LIBRARIES})
 ```
+
+# C++11
+
+Currently, c++11 is required to build matplotlib-cpp. The last working commit that did
+not have this requirement was `717e98e752260245407c5329846f5d62605eff08`.
+
+Note that support for c++98 was dropped more or less accidentally, so if you have to work
+with an ancient compiler and still want to enjoy the latest additional features, I'd
+probably merge a PR that restores support.
+
 # Python 3
 
 This library supports both python2 and python3 (although the python3 support is probably far less tested,

matplotlibcpp.h

@@ -6,11 +6,8 @@
 #include <algorithm>
 #include <stdexcept>
 #include <iostream>
-#include <stdint.h> // <cstdint> requires c++11 support
-
-#if __cplusplus > 199711L || _MSC_VER > 1800
-#  include <functional>
-#endif
+#include <cstdint> // <cstdint> requires c++11 support
+#include <functional>
 
 #include <Python.h>
 
@@ -1394,8 +1391,7 @@ inline void tight_layout() {
     Py_DECREF(res);
 }
 
-#if __cplusplus > 199711L || _MSC_VER > 1800
-// C++11-exclusive content starts here (variadic plot() and initializer list support)
+// Support for variadic plot() and initializer lists:
 
 namespace detail {
 
@@ -1524,6 +1520,4 @@ inline bool plot(const std::vector<double>& x, const std::vector<double>& y, con
     return plot<double>(x,y,keywords);
 }
 
-#endif
-
 } // end namespace matplotlibcpp