CMake 3.15 introduced a new way of handling MSVC CRT type definition for
the build: CMAKE_MSVC_RUNTIME_LIBRARY variable.
(https://cmake.org/cmake/help/latest/variable/CMAKE_MSVC_RUNTIME_LIBRARY.html)

This is supposed to be the way to go with MSVC CRT selection in new
projects. Using this method however breaks the current CMake script for
CUDA test. The reason is the CUDA test uses "FindCUDA" CMake module to
detect and set up CUDA support in CMake, which is deprecated since CMake
version 3.10, and which does not support CMAKE_MSVC_RUNTIME_LIBRARY
selector correctly (i.e. it does not propagate the compiler option
related to the CRT).

I did not find a way to "patch" in the correct compiler options, so
(while knowing this feature is only available from CMake 3.15 on) I
decided to change also the way CUDA is handled and instead of using
FindCUDA, used enable_language. Apart from having some nice additional
side-effects, it also fixed the problem with CRT selection.

However, the propagation of the compiler options (and in particular the
options related to C++ standard selection) is still a bit flaky on
Windows+MSVC platform, so it had to be done manually.

The patch makes two things in parallel:

1) Introduces MSVC_BUILD_STATIC, which, together with CMake version >=
3.15, allows building static version of the 'fmt' lib (and all the
tests).

2) At the same time, for CMake >= 3.15 it switches handling of CUDA
support from (old) FindCUDA to (new) enable_language, to fix the
problems which the old method has with the new CRT selector for MSVC in
a new CMake.

Added a check for CUDA before enabling it.

Using VERSION_LESS instead of VERSION_GREATER_EQUAL

Since apparently VERSION_GREATER_EQUAL exists only from CMake 3.7, while
Android is using CMake 3.6.

Removed MSVC_RUNTIME_LIBRARY logic from the CMake file.

The static build can be set on the command line with CMake >= 3.15
by defining the policy and the CMAKE_MSVC_RUNTIME_LIBARY this way:

cmake -G <gen> <options>
    -DCMAKE_POLICY_DEFAULT_CMP0091=NEW
    -DMSVC_BUILD_STATIC=ON
    -DCMAKE_MSVC_RUNTIME_LIBRARY="MultiThreaded$<$<CONFIG:Debug>:Debug>"

When MSVC_BUILD_DEBUG is set the test 'posix-mock-test' is skipped as it
does not build with the static runtime.

These variables get set to /usr/lib and /usr/include , which is totally wrong in a cross compilation
environment.

Changed to standard pkgconfig values consistent with most other ones.

To reserve space for the type traits sharing the same name as the ones
from standard library.

string_view_metadata was introduced to make compiled format relocatable.
However, format string compilation is an optimization and therefore
adding overhead and extra complexity for relocation is undesirable.
In most cases the string will be either static or outlive compilation
and formatting and if it doesn't, it's possible to make compiled
representation relocatale by other means.