add flake.nix and fix issues with build system variable expansion

[km-clay]

I wrote a nix flake that builds thtk.

while I was working on it I found that the installed thtk.pc is broken. building current master with a simple /usr/local prefix:

prefix = /usr/local
exec_prefix =
includedir = /include
libdir = /lib64

Libs: -L -lthtk
Cflags: -I

pkg-config --cflags --libs thtk returns -I -L -lthtk, so consumers get no include path and no library path.

this is caused by thtk/CMakeLists.txt:30 calling configure_file() without passing @ONLY as an argument, so CMake substitutes both variable forms: @VAR@ and ${VAR}. The pkg-config variable references in thtk/thtk.pc.in get eaten by CMake eagerly expanding them.

also, the paths in thtk.pc.in were built as ${prefix}/@CMAKE_INSTALL_INCLUDEDIR, which assumes those variables are relative. replacing the path construction with @CMAKE_INSTALL_FULL_INCLUDEDIR handles absolute paths correctly. Same for the LIBDIR path construction.

Notably, the currently open nixpkgs thtk pull request includes a hack designed to work around this issue. Not entirely relevant, but wanted to point it out to prove that it's not just an issue with my environment. (I also based the derivation in the flake on the one in that PR)

This PR includes both the fixes for the build system and the nix flake.