configure: mention quoting requirements in the help text

Change-Id: Id79548b8458dfa25fef17cb25688738bb719e7c5
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

config_help.txt

@@ -9,6 +9,12 @@ ICU_PREFIX=/opt/icu42 ICU_LIBS="-licui18n -licuuc -licudata".
 It is also possible to manipulate any QMAKE_* variable, to amend the values
 from the mkspec for the build of Qt itself, e.g., QMAKE_CXXFLAGS+=-g3.
 
+Note that the *_LIBS* and QMAKE_* assignments manipulate lists, so items
+containing meta characters (spaces in particular) need to be quoted according
+to qmake rules. On top of that, the assignments as a whole need to be quoted
+according to shell rules. It is recommended to use single quotes for the inner
+quoting and double quotes for the outer quoting.
+
 Top-level installation directories:
   -prefix <dir> ...... The deployment directory, as seen on the target device.
                        [/usr/local/Qt-$QT_VERSION, $PWD if -developer-build]