I want to require a c library which was build by with the autotools. To be honest I have little to no idea how they work :/ (The library which I want to require is "https://github.com/p4lang/PI")
I have executed the ./configure etc. scripts and successfully installed it.
When I search my usr I find the library under /usr/local/lib/libpi.a and analogously the header files under /usr/local/include/PI.
I build my project with cmake and would like to have a cross platform solution with it. However I would be satisfied to use the pkg-config command.
Does anybody know what is the "correct" / "recommended" way to get cflags, or at least a variant in which I do not have to hard code the paths?
The involvement of the Autotools ends at the point where the built artifacts are installed on the system. Using those does not go through the Autotools.* This applies just as much when the installed artifacts are libraries and headers as when they are executables. There's nothing special or different about using Autotools-built programs or libraries.
I build my project with cmake and would like to have a cross platform solution with it. However I would be satisfied to use the pkg-config command.
Just like projects served by any other build system, Autotools projects can build and install pkg-config configuration files, or CMake macros, or whatever other bits and pieces they might think appropriate to assist users, but this is project-specific. The Autotools do not create such additional pieces of their own accord, but some Autotools-based projects do add them. And some don't, just like some CMake projects don't, and some projects with hand-rolled build systems don't, etc..
Does anybody know what is the "correct" / "recommended" way to get cflags, or at least a variant in which I do not have to hard code the paths?
Note that typically, for a library whose name you know, the only flags you might need are those specifying the location of the library headers and / or one specifying the location of the libraries themselves. Even these are unnecessary if the relevant pieces are installed in places that the compiler looks by default. Also these are generally not considered CFLAGS, per se. Terminology varies a bit, but the former is a preprocessor flag, and the latter is a link flag.
Since you're using CMake, you could consider writing CMake code to search likely directories for the wanted libraries and headers, and to set the results in suitable variables for other code to use. That's more of an Autotools-style approach, though. Alternatively, you could define a user-set variable by which the wanted location(s) can be specified to CMake. This assumes that the third-party project is not already providing something useful for the purpose. Or, licensing permitting, you could package the third-party library together with your own, so that you are in control of where it gets installed.
In the general case, however, this is simply something that people have to deal with themselves when they build software. Make life easier for them by providing good documentation of what your project's dependencies are, and of how to inform the build system of their locations, and make useful provisions for feeding that information into the build system.
*An exception could be asserted for use of libtool archives, which an Autotools project might install alongside regular libraries -- if one wanted to use those, they would directly or indirectly go through libtool. But in practice, that's only going to happen in another Autotools project.