Fix compatibility with GCC 4.8

Closes #2045

extras/CompileOptions.cmake

@@ -32,8 +32,18 @@ endif()
 if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
 	if((CMAKE_CXX_COMPILER_VERSION VERSION_GREATER 4.8) AND(NOT ${COVERAGE}))
 		add_compile_options(-g -Og)
-	else()
-		add_compile_options(-g -O0)
+	else() # GCC 4.8
+		add_compile_options(
+			-g
+			-O0 # GCC 4.8 doesn't support -Og
+			-Wno-shadow  # allow the same name for a function parameter and a member functions
+			-Wp,-w  # Disable preprocessing warnings (see below)
+		)
+		# GCC 4.8 doesn't support __has_include, so we need to help him
+		add_definitions(
+			-DARDUINOJSON_ENABLE_STD_STRING=1
+			-DARDUINOJSON_ENABLE_STD_STREAM=1
+		)
 	endif()
 
 	add_compile_options(

extras/tests/Helpers/Allocators.hpp

@@ -90,6 +90,10 @@ class AllocatorLog {
       append(entry);
   }
 
+  void clear() {
+    log_.str("");
+  }
+
   void append(const AllocatorLogEntry& entry) {
     for (size_t i = 0; i < entry.count(); i++)
       log_ << entry.str() << "\n";
@@ -165,7 +169,7 @@ class SpyingAllocator : public ArduinoJson::Allocator {
   }
 
   void clearLog() {
-    log_ = AllocatorLog();
+    log_.clear();
   }
 
   const AllocatorLog& log() const {

extras/tests/Misc/TypeTraits.cpp

@@ -212,3 +212,8 @@ TEST_CASE("Polyfills/type_traits") {
     CHECK(is_enum<double>::value == false);
   }
 }
+
+TEST_CASE("is_std_string") {
+  REQUIRE(is_std_string<std::string>::value == true);
+  REQUIRE(is_std_string<EmptyClass>::value == false);
+}