Merge #21423: build: Cleanups and follow ups after bumping Qt to 5.12.10

e67c012 doc: Update dependencies.md with a new Qt version (Hennadii Stepanov)
cc25f89 build: Cleanup libxkbcommon_postprocess_cmds (Hennadii Stepanov)
72fc043 build, qt: Drop redundant -lxcb-static flag (Hennadii Stepanov)
cba4a7e build, qt: Always test plugins/subdir before adding to search paths (Hennadii Stepanov)

Pull request description:

  1) Always test `plugins/subdir` before adding to search paths as the existence of each subdir is not guaranteed for all platforms:
    - bitcoin/bitcoin#21376 (comment)

  2) Drop redundant `-lxcb-static` flag as it has been already linked with `Qt5XcbQpa`:
    - bitcoin/bitcoin#21363 (comment)

  3) Cleanup `libxkbcommon_postprocess_cmds` as there is no `share/` directory in the staging one:
    - bitcoin/bitcoin#21376 (comment)
    - bitcoin/bitcoin#21376 (comment)

  4) Update `dependencies.md`

ACKs for top commit:
  fanquake:
    ACK e67c012

Tree-SHA512: 9113ee97d5e7424290778154d62a68af804ee82efedbbe9776a7f692104d65b07d151e9f7f1f98ec08d18f6d63efef3e44b207bee67ad913f5dbc4eddbb8ea41

Author: fanquake

build-aux/m4/bitcoin_qt.m4

@@ -123,7 +123,12 @@ AC_DEFUN([BITCOIN_QT_CONFIGURE],[
     _BITCOIN_QT_CHECK_STATIC_LIBS
 
     if test "x$qt_plugin_path" != x; then
-      QT_LIBS="$QT_LIBS -L$qt_plugin_path/platforms -L$qt_plugin_path/styles"
+      if test -d "$qt_plugin_path/platforms"; then
+        QT_LIBS="$QT_LIBS -L$qt_plugin_path/platforms"
+      fi
+      if test -d "$qt_plugin_path/styles"; then
+        QT_LIBS="$QT_LIBS -L$qt_plugin_path/styles"
+      fi
       if test -d "$qt_plugin_path/accessible"; then
         QT_LIBS="$QT_LIBS -L$qt_plugin_path/accessible"
       fi
@@ -146,7 +151,7 @@ AC_DEFUN([BITCOIN_QT_CONFIGURE],[
     elif test "x$TARGET_OS" = xlinux; then
       dnl workaround for https://bugreports.qt.io/browse/QTBUG-74874
       AX_CHECK_LINK_FLAG([-lxcb-shm], [QT_LIBS="-lxcb-shm $QT_LIBS"], [AC_MSG_ERROR([could not link against -lxcb-shm])])
-      _BITCOIN_QT_CHECK_STATIC_PLUGIN([QXcbIntegrationPlugin], [-lqxcb -lxcb-static])
+      _BITCOIN_QT_CHECK_STATIC_PLUGIN([QXcbIntegrationPlugin], [-lqxcb])
       AC_DEFINE(QT_QPA_PLATFORM_XCB, 1, [Define this symbol if the qt platform is xcb])
     elif test "x$TARGET_OS" = xdarwin; then
       AX_CHECK_LINK_FLAG([[-framework Carbon]],[QT_LIBS="$QT_LIBS -framework Carbon"],[AC_MSG_ERROR(could not link against Carbon framework)])

depends/packages/libxkbcommon.mk

@@ -27,6 +27,6 @@ define $(package)_stage_cmds
 endef
 
 define $(package)_postprocess_cmds
-  rm -rf share/man share/doc lib/*.la
+  rm lib/*.la
 endef
 

doc/dependencies.md

@@ -21,7 +21,7 @@ These are the dependencies currently used by Bitcoin Core. You can find instruct
 | PCRE |  |  |  |  | [Yes](https://github.com/bitcoin/bitcoin/blob/master/depends/packages/qt.mk) |
 | Python (tests) |  | [3.6](https://www.python.org/downloads) |  |  |  |
 | qrencode | [3.4.4](https://fukuchi.org/works/qrencode) |  | No |  |  |
-| Qt | [5.9.8](https://download.qt.io/official_releases/qt/) | [5.9.5](https://github.com/bitcoin/bitcoin/issues/20104) | No |  |  |
+| Qt | [5.12.10](https://download.qt.io/official_releases/qt/) | [5.9.5](https://github.com/bitcoin/bitcoin/issues/20104) | No |  |  |
 | SQLite | [3.32.1](https://sqlite.org/download.html) | [3.7.17](https://github.com/bitcoin/bitcoin/pull/19077) |  |  |  |
 | XCB |  |  |  |  | [Yes](https://github.com/bitcoin/bitcoin/blob/master/depends/packages/qt.mk) (Linux only) |
 | xkbcommon |  |  |  |  | [Yes](https://github.com/bitcoin/bitcoin/blob/master/depends/packages/qt.mk) (Linux only) |