Update CMake to C++14

[jmjatlanta]

This PR changes the CMakeLists.txt file to use C++14, and removes references to C++11.

This also fixes a POSIX / Windows incompatibility in a function that Windows does not use. (Update: see #132)

NOTE: Separate issue: To compile FC alone on Windows, the CMakeModules/FindBoost.cmake needs to be deleted. Compiling as part of bitshares-core ignores that file. FindBoost.cmake should probably be permanently removed once we handle the pthread/boost/fc issue.

[pmconrad]

Please rebase -i to get rid of the rolled back change.

[pmconrad]

Also see comment in core PR:

it would be good to have a small C++-14 only change in both fc and core to verify that the settings are complete and have the intended effec

[pmconrad]

I've got one - std::enable_if_t as used in optional API parameters exists since C++-14 only. :-/

[jmjatlanta]

Also see comment in core PR:

it would be good to have a small C++-14 only change in both fc and core to verify that the settings are complete and have the intended effec

capturing local variables in lambdas produced a warning in gcc when compiling with c++11 specified and c++14 not required. Warning disappears with these changes. Would you like something that breaks the c++11 build instead of just warning?