Bug Report: Broken install headers after PR #1986 file reorganization

[saustinp]

Title: t8_helper_functions/ not installed to include path after #1986

Labels: bug, install, build

Description
After installing t8code to a prefix (e.g., cmake --install .), the t8_helper_functions directory is absent from the install tree. Any #include <t8_helper_functions/...> (e.g., in t8_cmesh/t8_cmesh.cxx) fails at compile time for downstream users. This appears to have been introduced by the file reorganization in #1986. I discovered this bug while trying to build the example tutorials/features/t8_features_curved_meshes.cxx.

File: src/CMakeLists.txt, line 241–246

Problem: The t8_helper_functions directory is installed with DESTINATION PRIVATE_HEADER instead of DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}. PRIVATE_HEADER is a CMake keyword for framework private headers and does not install to the include/ directory. Every other subdirectory (e.g., t8_cmesh, t8_data, t8_eclass) correctly uses ${CMAKE_INSTALL_INCLUDEDIR}.

Suggested Fix:
In CMakeLists.txt:

Before (line 242):

install( DIRECTORY t8_helper_functions
DESTINATION PRIVATE_HEADER

After:

install( DIRECTORY t8_helper_functions
DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}

Impact:
Makes it impossible to build projects against an installed copy of t8code.

Environment:

OS: Ubuntu 24.04
Compiler: g++/gcc 14.3.0 (Ubuntu 14.3.0-12ubuntu1~24~ppa1)
MPI version: OpenMPI 5.0.8
t8code commit hash: f878a8e7d9f70097f74b7ec6d68953f81979ccbf
CMake options:

-DCMAKE_BUILD_TYPE=Release \
-DCMAKE_CXX_COMPILER=g++ \
-DCMAKE_C_COMPILER=gcc \
-DCMAKE_INSTALL_PREFIX=~/.local/t8code \
-DCMAKE_CXX_STANDARD=20 \
-DCMAKE_C_STANDARD=17 \
-DCMAKE_CXX_EXTENSIONS=OFF \
-DT8CODE_ENABLE_VTK=ON \
-DT8CODE_BUILD_AS_SHARED_LIBRARY=ON \
-DMPI_HEADER_FOUND=TRUE \
-DT8CODE_ENABLE_OCC=ON \

Estimated priority: High. Should be solved as soon as possible.

Additional context:
Reproduced while building tutorials/features/t8_features_curved_meshes.cxx against the installed headers.