cmake: Fix IWYU in nix by adding CMAKE_CXX_IMPLICIT_INCLUDE_DIRECTORIES

Without this change, building with -DMP_ENABLE_IWYU=ON and newer versions of
nixpkgs (after 2025-11-12) fails with mysterious errors that look like:

make[2]: *** [CMakeFiles/mputil.dir/build.make:79: CMakeFiles/mputil.dir/src/mp/util.cpp.o] Error 1

with no other error messages. The are no messages because cmake -E
__run_co_compile hides the output from IWYU and does not display anything itself.

The actual problem is missing include directories on the compiler command line,
and the fix here is to extend a workaround previously added for clang-tidy to
be used for IWYU as well.

Breakage seems to have been caused by
NixOS/nixpkgs@76a8ffa
from NixOS/nixpkgs#445095, which I found by bisecting
with --first-parent to a change between the following merge commit:

https://github.com/NixOS/nixpkgs/commits/de21549a840be528ef4763dd7dca2006caf9c11f (bad)
https://github.com/NixOS/nixpkgs/commits/c4d1151093af5d483d1900d9214a65e68bc21560 (good)

If you check out the bad commit above (or any later commit) and revert
NixOS/nixpkgs@76a8ffa
the problem disappears.

I suspect the problem happens because cmake is not handling -cxx-isystem
properly, but did not debug further.

Author: ryanofsky

CMakeLists.txt

@@ -90,13 +90,6 @@ if(MP_ENABLE_CLANG_TIDY)
     message(FATAL_ERROR "MP_ENABLE_CLANG_TIDY is ON but clang-tidy is not found.")
   endif()
   set(CMAKE_CXX_CLANG_TIDY "${CLANG_TIDY_EXECUTABLE}")
-
-  # Workaround for nix from https://gitlab.kitware.com/cmake/cmake/-/issues/20912#note_793338
-  # Nix injects header paths via $NIX_CFLAGS_COMPILE; CMake tags these as
-  # CMAKE_CXX_IMPLICIT_INCLUDE_DIRECTORIES and omits them from the compile
-  # database, so clang-tidy, which ignores $NIX_CFLAGS_COMPILE, can't find capnp
-  # headers. Setting them as standard passes them to clang-tidy.
-  set(CMAKE_CXX_STANDARD_INCLUDE_DIRECTORIES ${CMAKE_CXX_IMPLICIT_INCLUDE_DIRECTORIES})
 endif()
 
 option(MP_ENABLE_IWYU "Run include-what-you-use with the compiler." OFF)
@@ -111,6 +104,15 @@ if(MP_ENABLE_IWYU)
   endif()
 endif()
 
+if(MP_ENABLE_CLANG_TIDY OR MP_ENABLE_IWYU)
+  # Workaround for nix from https://gitlab.kitware.com/cmake/cmake/-/issues/20912#note_793338
+  # Nix injects header paths via $NIX_CFLAGS_COMPILE; CMake tags these as
+  # CMAKE_CXX_IMPLICIT_INCLUDE_DIRECTORIES and omits them from the compile
+  # database, so clang-tidy, which ignores $NIX_CFLAGS_COMPILE, can't find capnp
+  # headers. Setting them as standard passes them to clang-tidy.
+  set(CMAKE_CXX_STANDARD_INCLUDE_DIRECTORIES ${CMAKE_CXX_IMPLICIT_INCLUDE_DIRECTORIES})
+endif()
+
 include("cmake/compat_config.cmake")
 include("cmake/pthread_checks.cmake")
 include(GNUInstallDirs)