Value: Use variant of enable_if to fix MakeString/MakeRaw in GCC.
Value: Use variant of enable_if to fix MakeString/MakeRaw in GCC.

Tested on GCC 4.8, GCC 4.9, and clang 3.6.

333fred/ntcore,333fred/ntcore,robotdotnet/ntcore,robotdotnet/ntcore,wpilibsuite/ntcore,wpilibsuite/ntcore,robotdotnet/ntcore,333fred/ntcore,333fred/ntcore,wpilibsuite/ntcore,wpilibsuite/ntcore,robotdotnet/ntcore