Improve project files of libfuzzer tests

Change-Id: I7977beb1bbc142326a3cc48435b91ec7293e2cff
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

tests/libfuzzer/README

@@ -12,7 +12,9 @@ data which triggered the crash. You can then use this to debug and fix the calle
 
 To run a test with libFuzzer:
 
-1. Install libFuzzer, e.g. from the repositories of the Linux distribution you are using.
+1. Install clang 5.0 or later, e.g. from the repositories of the Linux distribution you are using.
+   Depending on the version of clang and the source you are installing from, you might have to
+   install libFuzzer for this version of clang explicitly.
 2. Make sure clang and clang++ from this version of clang are found in PATH.
 3. Configure Qt with
     -platform linux-clang -coverage trace-pc-guard

tests/libfuzzer/corelib/serialization/qxmlstream/qxmlstreamreader/readnext/readnext.pro

@@ -1,5 +1,5 @@
 QT -= gui
-CONFIG += c++11 console
+CONFIG += console
 CONFIG -= app_bundle
 SOURCES += main.cpp
-LIBS += -lFuzzer
+LIBS += -fsanitize=fuzzer

tests/libfuzzer/gui/text/qtextdocument/setHtml/setHtml.pro

@@ -1,4 +1,3 @@
 QT += widgets
-CONFIG += c++11
 SOURCES += main.cpp
-LIBS += -lFuzzer
+LIBS += -fsanitize=fuzzer