Don't export the -std=c++11 flag from the fmt target

(cherry picked from commit b9aaa507)

fmt/CMakeLists.txt

@@ -15,7 +15,9 @@ if (FMT_CPPFORMAT)
 endif ()
 
 # Starting with cmake 3.1 the CXX_STANDARD property can be used instead.
-target_compile_options(fmt PUBLIC ${CPP11_FLAG})
+# Note: Don't make -std=c++11 public or interface, since it breaks projects
+# that use C++14.
+target_compile_options(fmt PRIVATE ${CPP11_FLAG})
 if (FMT_PEDANTIC)
   target_compile_options(fmt PRIVATE ${PEDANTIC_COMPILE_FLAGS})
 endif ()