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

[EricWF]

Currently the fmt CMake target exports -std=c++11 as part of its interface; Meaning any project that links to fmt has the -std=c++11 added to its compilation. However this will override the -std=<ver> flag provided by that project. This breaks projects that use C++14 or C++1z. In general I think it's bad style for projects to force a specific dialect, or any compile flags, upon its users.

This patch fixes the issue by making -std=c++11 a private flag of fmt. This prevent it from infecting downstream users.

[vitaut]

Good catch and thanks for the fix! Merged.