putting headers into build directory at configure time (symlinks) - cmake

Question

I have a project with the following file structure (transitioning to CMake), with source and header files in the same directories:
root
+- module1
+- a.hpp
+- a.cpp
+- b.hpp
+- b.cpp
+ module1a
+- a.hpp
+- b.cpp
+- module2
+- a.hpp
+- a.cpp
Header files are included in each other as #include<projectname/module1/a.hpp>. How can I instruct CMake to install all headers into the build directory as this structure, prior to building the .cpp files? I was briefly looking at install(DIRECTORY "${CMAKE_SOURCE_DIR}/module1" DESTINATION "projectname/module1" FILES_MATCHING PATTERN "*.hpp") (from CMake install header files and maintain directory heirarchy) but it does install those headers into the build directory (nothing happens).
The headers are needed only at compile-time, not at runtime.
Can I get some hint how to achieve this?
EDIT: The directory layout is different in the source three than how #include directives include it. For example, consider #include<projectname/module1a/a.hpp>. include_directories will not work for this header. The same for #include<projectname/module1/a.hpp as root!=projectname. Headers must be copied/symlinked from the old layout to the new layout first.

Answer 1

This is the solution (perhaps not perfect, but working) which will create symlinks at configure-time:
macro(create_symlink target linkname)
execute_process(
COMMAND ln -sf "${target}" "${linkname}"
RESULT_VARIABLE HEADER_LINK_STATUS
ERROR_VARIABLE HEADER_LINK_ERROR
)
if(NOT "${HEADER_LINK_STATUS}" EQUAL 0)
message(FATAL_ERROR "Symlinking headers failed:\n${HEADER_LINK_ERROR}")
endif()
endmacro()
MESSAGE(STATUS "Symlinking headers …")
FILE(MAKE_DIRECTORY "${CMAKE_BINARY_DIR}/projectname")
create_symlink("${CMAKE_SOURCE_DIR}/module1" "${CMAKE_BINARY_DIR}/projectname/module1")
create_symlink("${CMAKE_SOURCE_DIR}/module1/module1a" "${CMAKE_BINARY_DIR}/projectname/module1a")
create_symlink("${CMAKE_SOURCE_DIR}/module2" "${CMAKE_BINARY_DIR}/projectname/module2")