Fix standard default installation target presence

[FrancoisCarouge]

Presently, for usage, installation, and packaging of the fmt library as transient dependency from third party CMake project, the extraneous FMT_INSTALL option needs to be set, this option is unexpected, needs to be discovered by users.

FetchContent_Declare(
  fmt
  GIT_REPOSITORY "https://github.com/fmtlib/fmt"
  FIND_PACKAGE_ARGS NAMES fmt)

# Note the non-standard required configuration option setting for installing, packaging the dependency:
option(FMT_INSTALL "Enable installation for the {fmt} project." ON)

FetchContent_MakeAvailable(fmt)

The CMake installation target for the fmt library should be present by default to conform to CMake defaults. Standard installation support simplifies usage in projects. The presence of the target, by default, allows standard CMake transient usage, installation, and packaging of the fmt library in other projects.

The extraneous configuration option can now be removed from projects using the library. Transient usage, packaging, installation follows. It simplifies usage. Users need not discover this option anymore. The library works like others. It would not override pre-installed, or pinned fmt library versions by downstream projects from CMake's "first to record, wins" approach which is what allows hierarchical projects to have parent projects override content details of child projects. Now, regular usage just works:

FetchContent_Declare(
  fmt
  GIT_REPOSITORY "https://github.com/fmtlib/fmt"
  FIND_PACKAGE_ARGS NAMES fmt)
FetchContent_MakeAvailable(fmt)

Reference: https://cmake.org/cmake/help/latest/module/FetchContent.html

[vitaut]

Thanks for the PR. I think it's reasonable to be consistent with CMake defaults.