CMake how to link subproject with external library

I have the following Structure of my Project:

CMakeLists.txt
cmake
  CMakeLists.txt
src
  server
  CMakeLists.txt
    impl
      server.cpp
    CMakeLitst.txt
    main.cpp

I am trying to include subproject src that’s depends in my case on PostgreSQL.
Cmake is configuring fine, but building subproject with error:
libpq-fe.h: No such file or directory.

PostgreSQL is finding ok and linking without subproject good.

These are the current CMakeLists.txt Files:

cmake/CmakeLists.txt:

FetchContent_Declare(ormpp
             GIT_REPOSITORY https://github.com/qicosmos/ormpp.git
             GIT_TAG        6f4f9dcb07487a4a7273eb9fa0d7535f6adf6963)


FetchContent_MakeAvailable(ormpp)

CMakeLists.txt:

find_package(PostgreSQL REQUIRED)
add_subdirectory(cmake)
add_subdirectory(src)

src/server/CMakeLists.txt:

add_executable(server main.cpp)
target_link_libraries(server PUBLIC PostgreSQL::PostgreSQL)

What do I have to change in my CMakeFiles to get subproject linking to work properly?

Thanks a lot in advance.

>Solution :

You need to add this:

target_include_directories(server SYSTEM "D:\postgreSQL\postgresql\include")

Or this, if you have multiple targets and just want to specify it once:

include_directories(SYSTEM "D:\postgreSQL\postgresql\include")

Ref: https://cmake.org/cmake/help/latest/command/target_include_directories.html