-Wsuggest-override is not supported by gcc before 5.0 (#283)

* -Wsuggest-override is not supported by gcc before 5.0

* GCC prior to 5.0 should ignore not only -Wnon-virtual-dtor but also -Weffc++, otherwise non-virtual destructor problems will still be reported.

* The `#pragma GCC diagnostic push' should be used before setting up the temporary environment.

cmake/cxxopts.cmake

@@ -74,8 +74,10 @@ function(cxxopts_enable_warnings)
         set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} /W2")
     elseif(CMAKE_CXX_COMPILER_ID MATCHES "[Cc]lang" OR CMAKE_CXX_COMPILER_ID MATCHES "GNU")
       if (CMAKE_CXX_COMPILER_ID MATCHES "GNU")
+          if (${CMAKE_CXX_COMPILER_VERSION} VERSION_GREATER_EQUAL 5.0)
             set(COMPILER_SPECIFIC_FLAGS "-Wsuggest-override")
-        endif()
+          endif()
+      endif()
 
         set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -Werror -Wextra -Wshadow -Weffc++ -Wsign-compare -Wshadow -Wwrite-strings -Wpointer-arith -Winit-self -Wconversion -Wno-sign-conversion ${COMPILER_SPECIFIC_FLAGS}")
     endif()

include/cxxopts.hpp

@@ -288,8 +288,9 @@ namespace cxxopts
 #if defined(__GNUC__)
 // GNU GCC with -Weffc++ will issue a warning regarding the upcoming class, we want to silence it:
 // warning: base class 'class std::enable_shared_from_this<cxxopts::Value>' has accessible non-virtual destructor
-#pragma GCC diagnostic ignored "-Wnon-virtual-dtor"
 #pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wnon-virtual-dtor"
+#pragma GCC diagnostic ignored "-Weffc++"
 // This will be ignored under other compilers like LLVM clang.
 #endif
   class Value : public std::enable_shared_from_this<Value>