Add compile features for cmake 3.8+

CMakeLists.txt

@@ -158,6 +158,11 @@ if (FMT_PEDANTIC)
   target_compile_options(fmt PRIVATE ${PEDANTIC_COMPILE_FLAGS})
 endif ()
 
+if (NOT ${CMAKE_VERSION} VERSION_LESS 3.8)
+  target_compile_features(fmt INTERFACE cxx_std_11)
+endif ()
+
+
 target_include_directories(fmt PUBLIC
   $<BUILD_INTERFACE:${PROJECT_SOURCE_DIR}/include>
   $<INSTALL_INTERFACE:include>)
@@ -180,6 +185,10 @@ add_library(fmt::fmt-header-only ALIAS fmt-header-only)
 
 target_compile_definitions(fmt-header-only INTERFACE FMT_HEADER_ONLY=1)
 
+if (NOT ${CMAKE_VERSION} VERSION_LESS 3.8)
+  target_compile_features(fmt-header-only INTERFACE cxx_std_11)
+endif ()
+
 target_include_directories(fmt-header-only INTERFACE
   $<BUILD_INTERFACE:${PROJECT_SOURCE_DIR}/include>
   $<INSTALL_INTERFACE:include>)