Support generation of a pkg-config redox.pc file

[mpoquet]

Hello,

This PR adds the generation of a pkg-config file for redox when calling make/ninja/etc. install.

In brief, pkg-config eases the utilization of software libraries by allowing the developers of a library to define how the library should be used — instead of letting end users struggling with it. It therefore allows redox users to link/include it without having to know internal details such as include path, the library name or the dependencies that should also be included/linked.

Some build systems support the definition of dependencies via pkg-config (e.g., this is the main way to define dependencies in Meson, CMake can use pkg-config dependencies).

The following two examples show how a pkg-config redox file could help users by compiling the example located in examples/basic.cpp — after installing the new redox version.

Makefile usage example

basic: basic.cpp
	g++ -o basic `pkg-config --cflags --libs redox` basic.cpp

Meson usage example

project('basic', 'cpp')
redox = dependency('redox')
executable('basic', 'basic.cpp', dependencies: redox)