Provide a polyfill over std::to_string

Android apparently does not support `std::to_string`, so we add a
small polyfill over it. Right now only the ULP matcher uses it,
but we have had plans to use it in `StringMaker<int>` and friends,
as it performs a lot better than `std::stringstream` based
stringification on MSVC.

See #1280 for more details

CMakeLists.txt

@@ -187,6 +187,7 @@ set(INTERNAL_HEADERS
         ${HEADER_DIR}/internal/catch_test_spec_parser.h
         ${HEADER_DIR}/internal/catch_text.h
         ${HEADER_DIR}/internal/catch_timer.h
+        ${HEADER_DIR}/internal/catch_to_string.hpp
         ${HEADER_DIR}/internal/catch_tostring.h
         ${HEADER_DIR}/internal/catch_totals.h
         ${HEADER_DIR}/internal/catch_uncaught_exceptions.h

docs/configuration.md

@@ -99,6 +99,19 @@ This means that defining `CATCH_CONFIG_DEFAULT_REPORTER` to `"console"`
 is equivalent with the out-of-the-box experience.
 
 
+## C++11 toggles
+
+    CATCH_CONFIG_CPP11_TO_STRING // Use `std::to_string`
+
+Because we support platforms whose standard library does not contain
+`std::to_string`, it is possible to force Catch to use a workaround
+based on `std::stringstream`. On platforms other than Android,
+the default is to use `std::to_string`. On Android, the default is to
+use the `stringstream` workaround. As always, it is possible to override
+Catch's selection, by defining either `CATCH_CONFIG_CPP11_TO_STRING` or
+`CATCH_CONFIG_NO_CPP11_TO_STRING`.
+
+
 ## C++17 toggles
 
     CATCH_CONFIG_CPP17_UNCAUGHT_EXCEPTIONS  // Use std::uncaught_exceptions instead of std::uncaught_exception

include/internal/catch_compiler_capabilities.h

@@ -76,7 +76,11 @@
 #       define CATCH_CONFIG_COLOUR_NONE
 #endif
 
-
+////////////////////////////////////////////////////////////////////////////////
+// Android somehow still does not support std::to_string
+#if defined(__ANDROID__)
+#    define CATCH_INTERNAL_CONFIG_NO_CPP11_TO_STRING
+#endif
 
 ////////////////////////////////////////////////////////////////////////////////
 // Not all Windows environments support SEH properly
@@ -146,6 +150,10 @@
 #   define CATCH_CONFIG_WCHAR
 #endif
 
+#if !defined(CATCH_INTERNAL_CONFIG_NO_CPP11_TO_STRING) && !defined(CATCH_CONFIG_NO_CPP11_TO_STRING) && !defined(CATCH_CONFIG_CPP11_TO_STRING)
+#    define CATCH_CONFIG_CPP11_TO_STRING
+#endif
+
 #if defined(CATCH_INTERNAL_CONFIG_CPP17_UNCAUGHT_EXCEPTIONS) && !defined(CATCH_CONFIG_NO_CPP17_UNCAUGHT_EXCEPTIONS) && !defined(CATCH_CONFIG_CPP17_UNCAUGHT_EXCEPTIONS)
 #  define CATCH_CONFIG_CPP17_UNCAUGHT_EXCEPTIONS
 #endif

include/internal/catch_matchers_floating.cpp

@@ -6,6 +6,7 @@
  */
 
 #include "catch_matchers_floating.h"
+#include "catch_to_string.hpp"
 #include "catch_tostring.h"
 
 #include <cstdlib>
@@ -115,7 +116,7 @@ namespace Floating {
     }
 
     std::string WithinUlpsMatcher::describe() const {
-        return "is within " + std::to_string(m_ulps) + " ULPs of " + ::Catch::Detail::stringify(m_target) + ((m_type == FloatingPointKind::Float)? "f" : "");
+        return "is within " + Catch::to_string(m_ulps) + " ULPs of " + ::Catch::Detail::stringify(m_target) + ((m_type == FloatingPointKind::Float)? "f" : "");
     }
 
 }// namespace Floating

include/internal/catch_to_string.hpp

@@ -0,0 +1,28 @@
+/*
+ *  Created by Martin on 9/5/2018.
+ *
+ *  Distributed under the Boost Software License, Version 1.0. (See accompanying
+ *  file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+ */
+#ifndef TWOBLUECUBES_CATCH_TO_STRING_H_INCLUDED
+#define TWOBLUECUBES_CATCH_TO_STRING_H_INCLUDED
+
+#include <string>
+
+#include "catch_compiler_capabilities.h"
+#include "catch_stream.h"
+
+namespace Catch {
+    template <typename T>
+    std::string to_string(T const& t) {
+#if defined(CATCH_CONFIG_CPP11_TO_STRING)
+        return std::to_string(t);
+#else
+        ReusableStringStream rss;
+        rss << t;
+        return rss.str();
+#endif
+    }
+} // end namespace Catch
+
+#endif // TWOBLUECUBES_CATCH_TO_STRING_H_INCLUDED