build: install .pc and .cmake files to share/ (#3619)
build: install .pc and .cmake files to share/ (#3619)

As nlohmann_json is a header-only library, its pkg-config and cmake
config files should be installed to `share/` (GNUInstallDirs' DATADIR),
as `share/` is the canonical directory where architecture-independent
files should be stored, while `lib/` is for architecture-dependent stuff
(see the [FHS][]).

Apart from being technically correct, installing to `share/` can help
with cross-compiling, for example in Debian-based environments. If
you're interested, I'd suggest reading this [thread][].

Related to #1697

[FHS]: https://refspecs.linuxfoundation.org/FHS_3.0/fhs-3.0.html#usrshareArchitectureindependentData
[thread]: https://github.com/marzer/tomlplusplus/pull/165#issuecomment-1193146210
nlohmann/json,nlohmann/json,nlohmann/json