Allow installation via Meson

[mpoquet]

Hello and thanks for your work!

This commit allows to install the library via meson.

• The single_include header file is installed into lib/include/nlohmann/json.hpp.
• A pkg-config file nlohmann_json.pc is generated into the default directory lib/pkgconfig.

Usage example

meson build # [--prefix=/desired/installation/path]
cd build
ninja install

Example of generated nlohmann_json.pc

prefix=/usr/local
libdir=${prefix}/lib
includedir=${prefix}/include

Name: nlohmann_json
Description: JSON for Modern C++
Version: 3.4.0
Cflags: -I${includedir}

[coveralls]

Coverage remained the same at 100.0% when pulling ffe0898 on mpoquet:feature/meson-install-pkgconfig into d2e6e1b on nlohmann:develop.

[nlohmann]

Looks good to me.

[nlohmann]

Thanks!

[elvisoric]

Hello there.
Meson build script should be changed to allow installation only if the library is not used as subproject. I added issue #1463 and pull request regarding the issue.