qdoc: Don't add clang lib dir in system location to RPATH

If clang is installed in e.g. /usr/lib, then a qdoc binary built with
-developer-build will try to link against libQt5Core.so in /usr/lib and not the
one that's in the build directory.

Change-Id: I5e0e90ec96fd1f84a4dc904a35eec994d639bc9f
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>

src/qdoc/qdoc.pro

@@ -14,7 +14,7 @@ qtHaveModule(qmldevtools-private) {
 LIBS += $$CLANG_LIBS
 !contains(QMAKE_DEFAULT_INCDIRS, $$CLANG_INCLUDEPATH): INCLUDEPATH += $$CLANG_INCLUDEPATH
 
-!disable_external_rpath: QMAKE_RPATHDIR += $$CLANG_LIBDIR
+!contains(QMAKE_DEFAULT_LIBDIRS, $$CLANG_LIBDIR):!disable_external_rpath: QMAKE_RPATHDIR += $$CLANG_LIBDIR
 DEFINES += $$shell_quote(CLANG_RESOURCE_DIR=\"$${CLANG_LIBDIR}/clang/$${CLANG_VERSION}/include\")
 
 DEFINES += \