fix llvm libdir handling wrt RPATH, more or less properly

we must not add the llvm libdir if it's a system path. this was already
done in some places, but not in others.

while we're at it, re-shuffle some pre-existing conditionals to make
things consistent.

Task-number: QTCREATORBUG-20178
Change-Id: Ib7e5a81705494e4cf2f83a4782ecd0832b91e511
Reviewed-by: Marco Bubke <marco.bubke@qt.io>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>

src/tools/clangrefactoringbackend/clangrefactoringbackend.pro

@@ -23,7 +23,7 @@ QMAKE_CXXFLAGS += $$LLVM_CXXFLAGS
 SOURCES += \
     clangrefactoringbackendmain.cpp
 
-unix {
+unix:!disable_external_rpath:!contains(QMAKE_DEFAULT_LIBDIRS, $$LLVM_LIBDIR) {
     !osx: QMAKE_LFLAGS += -Wl,-z,origin
-    !disable_external_rpath: QMAKE_LFLAGS += -Wl,-rpath,$$shell_quote($${LLVM_LIBDIR})
+    QMAKE_LFLAGS += -Wl,-rpath,$$shell_quote($${LLVM_LIBDIR})
 }