Mark Cups::Cups optional target when building for non-APPLE systems

Relax the dependency on Cups for PrintSupport module on non-APPLE
platforms. 30bb15e added the missing
dependency for the PrintSupportPrivate target, but made it required.
In general users not necessarly need the Cups found when linking
PrintSupport on non-APPLE platforms. So we may mark Cups optional.

We also need Cups as the required dependency when Qt is built
statically, since then QCupsPrinterSupportPlugin will require the
library to be found.

Amends 30bb15e

Pick-to: 6.5
Change-Id: Ia31d52f7beb9fab6ffa5123147a243f5be6345ef
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>
(cherry picked from commit cce81f2)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
(cherry picked from commit 848cc2b)

src/plugins/printsupport/cups/CMakeLists.txt

@@ -1,7 +1,12 @@
 # Copyright (C) 2022 The Qt Company Ltd.
 # SPDX-License-Identifier: BSD-3-Clause
 
-qt_find_package(Cups PROVIDED_TARGETS Cups::Cups)
+if(NOT APPLE AND BUILD_SHARED_LIBS)
+    set(mark_cups_optional MARK_OPTIONAL)
+else()
+    set(mark_cups_optional "")
+endif()
+qt_find_package(Cups PROVIDED_TARGETS Cups::Cups ${mark_cups_optional})
 
 #####################################################################
 ## QCupsPrinterSupportPlugin Plugin:

src/printsupport/configure.cmake

@@ -9,8 +9,13 @@
 
 #### Libraries
 
-qt_find_package(Cups PROVIDED_TARGETS Cups::Cups MODULE_NAME printsupport QMAKE_LIB cups)
-
+if(NOT APPLE AND BUILD_SHARED_LIBS)
+    set(mark_cups_optional MARK_OPTIONAL)
+else()
+    set(mark_cups_optional "")
+endif()
+qt_find_package(Cups PROVIDED_TARGETS Cups::Cups
+    MODULE_NAME printsupport QMAKE_LIB cups ${mark_cups_optional})
 
 #### Tests