folly: fix split outputs

Downstream derivations were picking up references to the `dev` output
through indirect `__FILE__` usage in the headers. I borrowed and
modified this trick from the Boost package, where I believe vcunat
originated it. It’s not very pretty and it should be in a hook or
something, but it works at a pinch.

pkgs/by-name/fo/folly/package.nix

@@ -7,6 +7,7 @@
   cmake,
   ninja,
   pkg-config,
+  removeReferencesTo,
 
   boost,
   double-conversion,
@@ -54,6 +55,7 @@ stdenv.mkDerivation (finalAttrs: {
     cmake
     ninja
     pkg-config
+    removeReferencesTo
   ];
 
   # See CMake/folly-deps.cmake in the Folly source tree.
@@ -113,6 +115,18 @@ stdenv.mkDerivation (finalAttrs: {
         '@CMAKE_INSTALL_FULL_INCLUDEDIR@'
   '';
 
+  postFixup = ''
+    # Sanitize header paths to avoid runtime dependencies leaking in
+    # through `__FILE__`.
+    (
+      shopt -s globstar
+      for header in "$dev/include"/**/*.h; do
+        sed -i "1i#line 1 \"$header\"" "$header"
+        remove-references-to -t "$dev" "$header"
+      done
+    )
+  '';
+
   passthru = {
     # folly-config.cmake, will `find_package` these, thus there should be
     # a way to ensure abi compatibility.