Missing build system dependencies for doc snippets results in broken build

The doc snippets assume they can build after the module they are documenting is done building, but that's not the case for static builds, which may depend on plugins such as the platform plugin being built first. This results in broken builds when building in parallell. Attempted quick fix in https://codereview.qt-project.org/c/qt/qtbase/+/308948, but was using the wrong names for the rules. The doc snippets should probably be part of the doc subdirectory, and set up to build once all the other required parts have been built. 

Ld Debug-iphonesimulator/testlib_cppsnippet.app/testlib_cppsnippet normal x86_64
cd /Users/torarne/build/qt/5.15-ios-test/qtbase/src/testlib/doc/snippets
export PATH="/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/usr/bin:/Applications/Xcode.app/Contents/Developer/usr/bin:/Users/torarne/.zinit/polaris/bin:/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin:/Applications/VMware Fusion.app/Contents/Public:/opt/X11/bin:/Library/Apple/usr/bin:/usr/local/sbin:/Users/torarne/dev/qt/repotools/bin"
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang++ -target x86_64-apple-ios12.0-simulator -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator13.7.sdk -L/Users/torarne/build/qt/5.15-ios-test/qtbase/src/testlib/doc/snippets/Debug-iphonesimulator -L/Users/torarne/build/qt/5.15-ios-test/qtbase/plugins/platforms -L/Users/torarne/build/qt/5.15-ios-test/qtbase/lib -L/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator13.7.sdk/System/Library/Frameworks -F/Users/torarne/build/qt/5.15-ios-test/qtbase/src/testlib/doc/snippets/Debug-iphonesimulator -filelist /Users/torarne/build/qt/5.15-ios-test/qtbase/src/testlib/doc/snippets/.obj/testlib_cppsnippet.build/Debug-iphonesimulator/testlib_cppsnippet.build/Objects-normal/x86_64/testlib_cppsnippet.LinkFileList -dead_strip -Xlinker -object_path_lto -Xlinker /Users/torarne/build/qt/5.15-ios-test/qtbase/src/testlib/doc/snippets/.obj/testlib_cppsnippet.build/Debug-iphonesimulator/testlib_cppsnippet.build/Objects-normal/x86_64/testlib_cppsnippet_lto.o -Xlinker -no_deduplicate -Xlinker -objc_abi_version -Xlinker 2 -stdlib=libc++ -Wl,-e,_qt_main_wrapper -Wl,-rpath,@executable_path/Frameworks -L/Users/torarne/build/qt/5.15-ios-test/qtbase/plugins/platforms -lqios_debug -framework MobileCoreServices -lm -framework UIKit -lz -framework OpenGLES -Xlinker -sectcreate -Xlinker __TEXT -Xlinker __entitlements -Xlinker /Users/torarne/build/qt/5.15-ios-test/qtbase/src/testlib/doc/snippets/.obj/testlib_cppsnippet.build/Debug-iphonesimulator/testlib_cppsnippet.build/testlib_cppsnippet.app-Simulated.xcent -lQt5Test_debug -framework Security -lQt5Sql_debug -lQt5Widgets_debug -lQt5Gui_debug -framework Metal -lqtlibpng_debug -lqtharfbuzz_debug -framework CoreText -framework CoreGraphics -lQt5Core_debug -framework CoreFoundation -framework Foundation -lqtpcre2_debug -Xlinker -dependency_info -Xlinker /Users/torarne/build/qt/5.15-ios-test/qtbase/src/testlib/doc/snippets/.obj/testlib_cppsnippet.build/Debug-iphonesimulator/testlib_cppsnippet.build/Objects-normal/x86_64/testlib_cppsnippet_dependency_info.dat -o /Users/torarne/build/qt/5.15-ios-test/qtbase/src/testlib/doc/snippets/Debug-iphonesimulator/testlib_cppsnippet.app/testlib_cppsnippet
ld: library not found for -lqios_debug
clang: error: linker command failed with exit code 1 (use -v to see invocation)

• • BUILD FAILED **