Cmake issue with building a small library - build

Question

I am currently trying to deepen my understanding regarding CMake. I try to use http://libqglviewer.com/introduction.html]LibQGLViewer as a Third-Party library in a C++ Project of mine.
The CMakeLists.txt in the associated subdirectory looks like the following where the part, I have questions is the add_libary section and some header and source files were omitted for clarity
cmake_minimum_required (VERSION 3.5.1 FATAL_ERROR)
set(target_name QGLViewerQt5)
project(${target_name})
message(STATUS "BUILDING QGLViewer-2.7.0 FROM SOURCE!")
set(BASE_DIR QGLViewer)
set(VRENDER_DIR VRender)
set(CMAKE_AUTOMOC ON)
set(QGLheaders
${BASE_DIR}/camera.h
${BASE_DIR}/config.h
${BASE_DIR}/${VRENDER_DIR}/AxisAlignedBox.h
${BASE_DIR}/${VRENDER_DIR}/Exporter.h
)
set(QGLsources
${BASE_DIR}/camera.cpp
${BASE_DIR}/${VRENDER_DIR}/Exporter.cpp
)
add_library(${target_name} ${QGLsources} ${QGLheaders})
target_include_directories(${target_name}
PUBLIC .
)
target_link_libraries(${target_name}
${OPENGL_LIBRARIES}
Qt5::Core
Qt5::Widgets
Qt5::Xml
Qt5::OpenGL
)
set(CMAKE_AUTOMOC OFF)
install(TARGETS ${target_name} DESTINATION lib)
My Application runs and everything is fine.
However, I read that one should only include the source files with add_library and then use target_include_directories to consider the associated header files. So I changes the above part to
add_library(${target_name} SHARED ${QGLsources})
target_include_directories(${target_name}
PUBLIC
${PROJECT_SOURCE_DIR}/QGLViewer
${PROJECT_SOURCE_DIR}/QGLViewer/VRender
)
but now, I get an error trying to make my project
fatal error: QGLViewer/qglviewer.h: No such file or directory
compilation terminated.
can you please tell me
What exactly does PUBLIC . do? I know what PUBLIC does but what does the point mean?
Why does it not work as before?
thank you in advance
PS: The folder structure is as follows

Answer 1

Firts one, target_include_directories() has following syntax according official docs target_include_directories(<target> [SYSTEM] [BEFORE]
<INTERFACE|PUBLIC|PRIVATE> [items1...] [<INTERFACE|PUBLIC|PRIVATE> [items2...] ...]) as you see there few keywords might be used:
PRIVATE: adds directories to INCLUDE_DIRECTORIES property of <target>
INTERFACE: adds directories to INTERFACE_INCLUDE_DIRECTORIES property of <target>
PUBLIC: adds directories to INCLUDE_DIRECTORIES and INTERFACE_INCLUDE_DIRECTORIES properties of <target>
What about those properties:
INCLUDE_DIRECTORIES - contains list of directories to search header files you used in project
INTERFACE_INCLUDE_DIRECTORIES - contains list of directories to search header files too, but it has transitivity, that means you able to inherit your include directories to project linked through target_link_libraries()
Second one, your code doesn't work because you are using another include folders paths, your source file must be contain something like #include <QGLViewer/qglviewer.h> but since you do not include root folder anymore (. in your previous code), but linking QGLViewer directly - you must type #include <qglviewer.h>. Fix it for each #include you are using or add ${PROJECT_SOURCE_DIR} to target_include_directories instead.