Modify the pfd third party import to prevent a CMake deprecation warning

We populate the project portable-file-dialogs and do not use that
project's CMakeLists.txt, whose cmake_minimum_required() call generates
a deprecation warning on CMake.

cmake/third_party/pfd.cmake

@@ -10,11 +10,16 @@ FetchContent_Declare(
     GIT_REPOSITORY https://github.com/samhocevar/portable-file-dialogs.git
     GIT_TAG 7f852d88a480020d7f91957cbcefe514fc95000c
 )
-FetchContent_MakeAvailable(pfd)
+FetchContent_Populate(pfd)
 
-# To make it visible in the IDE
-add_custom_target(pfd SOURCES
+add_library(pfd
+    INTERFACE
     ${pfd_SOURCE_DIR}/portable-file-dialogs.h
 )
 
+target_include_directories(pfd
+    INTERFACE
+    ${pfd_SOURCE_DIR}
+)
+
 set_property(TARGET pfd PROPERTY FOLDER "third_parties")

src/gui/CMakeLists.txt

@@ -51,7 +51,7 @@ target_link_libraries(picross_solver
     picross::picross
     picross::utils
     imgui
-    portable_file_dialogs
+    pfd
 )
 
 install(TARGETS picross_solver)