Add compile features for cmake 3.8+

[Lectem]

This pull requests adds the minimal requirements of fmtlib to the CMake targets.
This is needed so that the users don't have to do it on all their target linking with fmt::*.
A cmake version check is added since the cxx_std_11 compile feature is not available before CMake 3.8.

Note that without this feature required by the user, such an error can occur:

In file included from /usr/bin/../lib/gcc/x86_64-linux-gnu/6.5.0/../../../../include/c++/6.5.0/type_traits:35:/usr/bin/../lib/gcc/x86_64-linux-gnu/6.5.0/../../../../include/c++/6.5.0/bits/c++0x_warning.h:32:2: error: This file requires compiler and library support for the ISO C++ 2011 standard. This support must be enabled with the -std=c++11 or -std=gnu++11 compiler options.
#error This file requires compiler and library support \

I agree that my contributions are licensed under the {fmt} license, and agree to future changes to the licensing.

[vitaut]

Thanks for the PR. Some comments inline.

[Lectem]

I did the modifications to require only cxx_auto_type and cxx_variadic_templates.
As a note, I think we could also remove the cxx14.cmake helper and use Conditional Compilation Options instead. (For a later PR?)

Most of the current macros have a mapping:

Supported by Cmake 3.1

• SUPPORTS_VARIADIC_TEMPLATES > cxx_variadic_macros
• SUPPORTS_INITIALIZER_LIST > cxx_generalized_initializers
• SUPPORTS_ENUM_BASE > cxx_strong_enums (a bit more aggressive, but enum base and strongly typed enums are from the same proposal)
• SUPPORTS_USER_DEFINED_LITERALS > cxx_user_literals

Unsupported by Cmake 3.1

• SUPPORTS_TYPE_TRAITS

[vitaut]

Merged, thanks!

I think we could also remove the cxx14.cmake helper and use Conditional Compilation Options instead. (For a later PR?)

Good idea.

SUPPORTS_VARIADIC_TEMPLATES can just go away since variadic templates is a hard requirement now.