Add optional interop headers

Those headers are used for interoping with various popular C++
libraries. For now, those libraries are Eigen, GLM, HandmadeMath,
Raylib, and SFML. Some other ones may be added in the future.

Those are disabled by default, as most projects I would imagine would
not use them. But in case you need any of them for special reasons like
I did, they are there.

Signed-off-by: Slendi <slendi@socopon.com>

flake.nix

@@ -55,10 +55,14 @@
             })
           ];
 
+          extraCMakeFlags = [
+            "-DSMATH_INSTALL_INTEROP_HEADERS=ON"
+          ];
+
           installPhase = ''
             runHook preInstall
-            mkdir -p $out/include/smath
-            cp ../include/smath/smath.hpp $out/include/smath/
+            mkdir -p $out/include/
+            cp -r ../include/smath/ $out/include/
             runHook postInstall
           '';