fix: use absolute path for include_dir due to nested CMakeLists

Co-Authored-By: Claus Klein <1833050+ClausKlein@users.noreply.github.com>

Author: aminya

my_header_lib/CMakeLists.txt

@@ -2,9 +2,9 @@ add_library(my_header_lib INTERFACE)
 target_link_libraries(my_header_lib INTERFACE project_options project_warnings) # link project_options/warnings
 
 # Includes
-set(my_header_lib_INCLUDE_DIR "./include" CACHE STRING "") # must be relative paths
+set(my_header_lib_INCLUDE_DIR "${CMAKE_CURRENT_SOURCE_DIR}/include" CACHE STRING "") # must be relative paths
 target_include_directories(
-  my_header_lib INTERFACE "$<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/${my_header_lib_INCLUDE_DIR}>"
+  my_header_lib INTERFACE "$<BUILD_INTERFACE:${my_header_lib_INCLUDE_DIR}>"
                           "$<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>")
 
 # Find dependencies:

my_lib/CMakeLists.txt

@@ -2,12 +2,16 @@ add_library(my_lib "./src/my_lib/lib.cpp")
 target_link_libraries(my_lib PRIVATE project_options project_warnings) # link project_options/warnings
 
 # Includes
-set(my_lib_INCLUDE_DIR "include" CACHE STRING "") # must be relative paths
-target_include_directories(my_lib PUBLIC "$<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/${my_lib_INCLUDE_DIR}>"
+set(my_lib_INCLUDE_DIR
+    "${CMAKE_CURRENT_SOURCE_DIR}/include"
+    CACHE STRING "")
+target_include_directories(my_lib PUBLIC "$<BUILD_INTERFACE:${my_lib_INCLUDE_DIR}>"
                                          "$<INSTALL_INTERFACE:./${CMAKE_INSTALL_INCLUDEDIR}>")
 
 # Find dependencies:
-set(my_lib_DEPENDENCIES_CONFIGURED fmt CACHE STRING "")
+set(my_lib_DEPENDENCIES_CONFIGURED
+    fmt
+    CACHE STRING "")
 
 foreach(DEPENDENCY ${my_lib_DEPENDENCIES_CONFIGURED})
   find_package(${DEPENDENCY} CONFIG REQUIRED)