cmake: Copy Qt platform plugins on Windows

Fixes #1.

cmake/install_windows.cmake

@@ -12,6 +12,8 @@ install(
 file(GLOB DLLS "${CMAKE_BINARY_DIR}/*.dll")
 install(FILES ${DLLS} DESTINATION .)
 
+install(DIRECTORY "${CMAKE_BINARY_DIR}/platforms" DESTINATION .)
+
 install(
     DIRECTORY "${CMAKE_BINARY_DIR}/tessdata"
     DESTINATION .

cmake/install_windows_msys2.cmake

@@ -22,7 +22,24 @@ endfunction()
 copy_tessdata("${CMAKE_BINARY_DIR}")
 
 
+function(copy_qt5_plugins dst_dir)
+    set(SRC_DIR "$ENV{MINGW_PREFIX}/share/qt5/plugins")
+
+    add_custom_command(
+        OUTPUT "${dst_dir}/platforms"
+        COMMAND ${CMAKE_COMMAND} -E make_directory "${dst_dir}/platforms"
+        COMMAND ${CMAKE_COMMAND} -E copy "${SRC_DIR}/platforms/qwindows.dll" "${dst_dir}/platforms"
+        DEPENDS "${SRC_DIR}"
+        VERBATIM
+    )
+
+    add_custom_target("qt5_plugins" ALL DEPENDS "${dst_dir}/platforms")
+endfunction()
+
+
 if (DPSO_UI STREQUAL "qt")
+    copy_qt5_plugins("${CMAKE_BINARY_DIR}")
+
     include(qt_utils)
     include(get_linguas)
 

doc/building-windows-msys2.txt

@@ -162,6 +162,8 @@ necessary data files:
   * "locale" dir - compiled gettext catalogs. Only created if the
     DPSO_COMPILE_PO is enabled.
 
+  * "platforms" dir - platform-specific Qt plugins
+
   * "tessdata" dir - Tesseract data files. CMake will copy that
     directory from "$MINGW_PREFIX/share", and then remove "osd" and
     "equ" traineddata files, which are not used by dpScreenOCR.
@@ -177,8 +179,9 @@ section below.
 2.2 Copying DLLs
 ----------------
 
-CMake doesn't copy DLL files, so you have to do this yourself with the
-following command:
+CMake only copies DLL files from the "$MINGW_PREFIX/share/qt5/plugins"
+directory. Other DLLs need to be copied manually with the following
+command:
 
     cp $(sed -e "s:^:$MINGW_PREFIX/bin/:" ../doc/msys2-dlls.txt) .