Allow using system libfmt

[jeandudey]

Some distributions such as GNU Guix don't allow using bundled dependencies and code, so I'm adding the option to use a system version of libfmt instead of using the bundled one.

Had to nest fmt and CL in another sub-directory of the same name to avoid accidental inclusion of the fmt headers.

[SChernykh]

don't allow using bundled dependencies and code

How do they not allow it? It just doesn't compile? What is the error?

[jeandudey]

How do they not allow it? It just doesn't compile? What is the error?

@SChernykh the GNU Guix project generally tries to remove bundled dependencies from packages if it's available already, for example, fmt is already packaged and it the library tests are run after building it, libraries are also un-bundled for auditability and to avoid re-building code that's already built.

For example, on the package definition the hwloc library is unbundled as it is available on GNU Guix and xmrig supports building without it being on the src/3rdparty directory, other libraries such as OpenCL headers and libfmt are also available but require changes in xmrig first to avoid patching large chunks of the source code in the package definition.

This is done so that GNU Guix creates a source archive of the xmrig source code but doesn't contain bundled code in the src/3rdparty directory.

[SChernykh]

In this case it should have CMakeLists option for libfmt, just like hwloc has WITH_HWLOC. Also, the PR should be made to dev branch, not master.

Edit: this option should be called something like WITH_BUNDLED_FMT and default to ON because it doesn't just turn off fmt.

[jeandudey]

Added the option in f0f6d16.

(made fixup commit to squash once the review is done)