When the testsuite is enabled, the "cmake_add_subdirectory" test
adds a second copy of the project into the build configuration, and the
project files are installed twice.
This becomes super problematic when it tries to install a file from
CMAKE_BINARY_DIR which is only available in CMAKE_CURRENT_BINARY_DIR
and bombs out with the following error message:
```
[...]
-- Installing: <DESTDIR>/usr/lib/cmake/nlohmann_json/nlohmann_jsonTargets.cmake
CMake Error at test/cmake_add_subdirectory/nlohmann_json/cmake_install.cmake:73 (file):
file INSTALL cannot find
"......./nlohmann-json/builddir/test/cmake_add_subdirectory/nlohmann_json.pc":
No such file or directory.
Call Stack (most recent call first):
test/cmake_add_subdirectory/cmake_install.cmake:47 (include)
test/cmake_install.cmake:49 (include)
cmake_install.cmake:94 (include)
```
Also call out to the guidelines for using pkg-config dependencies first,
and reference it for other build systems as well.
Although the possibility of installing with a pkg-config file is
somewhat hidden away in the meson docs, it's been deemed less invasive
due to not distracting away from cmake. So it will have to do.
This serves as a minimal release-only way to embed json into a project.
Add meson support to this directly, to make it usable standalone as a
meson subproject.
Implements #1672
Eli Schwartz