CMake: fix absolute include install dir

Users may want to provide absolute paths in `CMAKE_INSTALL_INCLUDEDIR`.
When this is the case, we should not prefix it in any way.

Author: nim65s

install/CMakeLists.txt

@@ -17,6 +17,14 @@ endforeach()
 # Install package config files
 #
 
+if(IS_ABSOLUTE CMAKE_INSTALL_INCLUDEDIR)
+  set(CMAKE_INSTALL_INCLUDEDIR_EXPORTS "${CMAKE_INSTALL_INCLUDEDIR}")
+  set(CMAKE_INSTALL_INCLUDEDIR_PC "${CMAKE_INSTALL_INCLUDEDIR}")
+else()
+  set(CMAKE_INSTALL_INCLUDEDIR_EXPORTS "\${_IMPORT_PREFIX}/${CMAKE_INSTALL_INCLUDEDIR}")
+  set(CMAKE_INSTALL_INCLUDEDIR_PC "\${prefix}/${CMAKE_INSTALL_INCLUDEDIR}")
+endif()
+
 configure_file("${CMAKE_CURRENT_SOURCE_DIR}/zenohcxx.pc.in" "${CMAKE_CURRENT_BINARY_DIR}/zenohcxx.pc" @ONLY)
 if(APPLE OR UNIX)
 	install(FILES "${CMAKE_CURRENT_BINARY_DIR}/zenohcxx.pc" DESTINATION "${CMAKE_INSTALL_LIBDIR}/pkgconfig" OPTIONAL COMPONENT dev)

install/PackageConfig.cmake.in

@@ -25,15 +25,15 @@ endif()
 
 if(NOT TARGET zenohcxx)
 	add_library(zenohcxx INTERFACE IMPORTED)
-	target_include_directories(zenohcxx INTERFACE "${_IMPORT_PREFIX}/@CMAKE_INSTALL_INCLUDEDIR@")
+	target_include_directories(zenohcxx INTERFACE "@CMAKE_INSTALL_INCLUDEDIR_EXPORTS@")
 endif()
 
 # zenohcxx for zenohpico
 if(TARGET zenohpico::lib AND NOT TARGET zenohcxx_zenohpico)
 	message(STATUS "defined lib target zenohcxx::zenohpico for zenohpico::lib")
 	add_library(zenohcxx_zenohpico INTERFACE IMPORTED)
 	target_compile_definitions(zenohcxx_zenohpico INTERFACE ZENOHCXX_ZENOHPICO)
-    target_include_directories(zenohcxx_zenohpico INTERFACE "${_IMPORT_PREFIX}/@CMAKE_INSTALL_INCLUDEDIR@")
+	target_include_directories(zenohcxx_zenohpico INTERFACE "@CMAKE_INSTALL_INCLUDEDIR_EXPORTS@")
 	add_dependencies(zenohcxx_zenohpico zenohpico::lib)
 	target_link_libraries(zenohcxx_zenohpico INTERFACE zenohpico::lib)
 	add_library(zenohcxx::zenohpico ALIAS zenohcxx_zenohpico)
@@ -44,7 +44,7 @@ if(TARGET zenohc::lib AND NOT TARGET zenohcxx_zenohc)
 	message(STATUS "defined lib target zenohcxx::zenohc for zenohc::lib")
 	add_library(zenohcxx_zenohc INTERFACE IMPORTED)
 	target_compile_definitions(zenohcxx_zenohc INTERFACE ZENOHCXX_ZENOHC)
-    target_include_directories(zenohcxx_zenohc INTERFACE "${_IMPORT_PREFIX}/@CMAKE_INSTALL_INCLUDEDIR@")
+	target_include_directories(zenohcxx_zenohc INTERFACE "@CMAKE_INSTALL_INCLUDEDIR_EXPORTS@")
 	add_dependencies(zenohcxx_zenohc zenohc::lib)
 	target_link_libraries(zenohcxx_zenohc INTERFACE zenohc::lib)
 	add_library(zenohcxx::zenohc ALIAS zenohcxx_zenohc)

install/zenohcxx.pc.in

@@ -4,4 +4,4 @@ Name: @PROJECT_NAME@
 Description: @CMAKE_PROJECT_DESCRIPTION@
 URL: @CMAKE_PROJECT_HOMEPAGE_URL@
 Version: @PROJECT_VERSION@
-Cflags: -I${prefix}/@CMAKE_INSTALL_INCLUDEDIR@
+Cflags: -I@CMAKE_INSTALL_INCLUDEDIR_PC@