fix c++98 being detected as newer than c++11

CMakeLists.txt

@@ -22,7 +22,6 @@ endif()
 
 add_library(ring-buffer INTERFACE)
 target_include_directories(ring-buffer INTERFACE include/)
-target_compile_features(ring-buffer INTERFACE cxx_std_11)
 
 if(RING_BUFFER_CONSTEXPR)
     target_compile_definitions(ring-buffer INTERFACE RING_BUFFER_CONSTEXPR)

detect/cxxstd.cpp

@@ -0,0 +1,5 @@
+#if __cpluplus < CXX_STANDARD
+#error "c++ standard not new enough"
+#endif
+
+int main() {}

detect/features.cmake

@@ -1,9 +1,21 @@
-if(${CMAKE_CXX_STANDARD} GREATER_EQUAL 11)
+try_compile(cxx-least-11 ${CMAKE_BINARY_DIR}/detect ${CMAKE_SOURCE_DIR}/detect/cxxstd.cpp
+    COMPILE_DEFINITIONS CXX_STANDARD=201103L
+    CMAKE_CXX_STANDARD ${CMAKE_CXX_STANDARD}
+    CXX_STANDARD_REQUIRED ${CMAKE_CXX_STANDARD_REQUIRED}
+    CXX_EXTENSIONS ${CMAKE_CXX_EXTENSIONS}
+)
+if(cxx-least-11)
     set(RING_BUFFER_NOEXCEPT ON)
     message(STATUS "Enabling RING_BUFFER_NOEXCEPT (C++11 and up)")
 endif()
 
-if(${CMAKE_CXX_STANDARD} GREATER_EQUAL 14)
+try_compile(cxx-least-14 ${CMAKE_BINARY_DIR}/detect ${CMAKE_SOURCE_DIR}/detect/cxxstd.cpp
+    COMPILE_DEFINITIONS CXX_STANDARD=201402L
+    CMAKE_CXX_STANDARD ${CMAKE_CXX_STANDARD}
+    CXX_STANDARD_REQUIRED ${CMAKE_CXX_STANDARD_REQUIRED}
+    CXX_EXTENSIONS ${CMAKE_CXX_EXTENSIONS}
+)
+if(cxx-least-14)
     set(RING_BUFFER_CONSTEXPR ON)
     message(STATUS "Enabling RING_BUFFER_CONSTEXPR (C++14 and up)")
 endif()