build: fix typo in configure help message

--with-qt-libdir etc, not --with-gui-libdir.
The typo was introduced in a9dbcf036b.

src/m4/bitcoin_qt.m4

@@ -59,10 +59,10 @@ AC_DEFUN([BITCOIN_QT_INIT],[
     ],
     [bitcoin_qt_want_version=auto])
 
-  AC_ARG_WITH([qt-incdir],[AS_HELP_STRING([--with-gui-incdir=INC_DIR],[specify qt include path (overridden by pkgconfig)])], [qt_include_path=$withval], [])
-  AC_ARG_WITH([qt-libdir],[AS_HELP_STRING([--with-gui-libdir=LIB_DIR],[specify qt lib path (overridden by pkgconfig)])], [qt_lib_path=$withval], [])
-  AC_ARG_WITH([qt-plugindir],[AS_HELP_STRING([--with-gui-plugindir=PLUGIN_DIR],[specify qt plugin path (overridden by pkgconfig)])], [qt_plugin_path=$withval], [])
-  AC_ARG_WITH([qt-bindir],[AS_HELP_STRING([--with-gui-bindir=BIN_DIR],[specify qt bin path])], [qt_bin_path=$withval], [])
+  AC_ARG_WITH([qt-incdir],[AS_HELP_STRING([--with-qt-incdir=INC_DIR],[specify qt include path (overridden by pkgconfig)])], [qt_include_path=$withval], [])
+  AC_ARG_WITH([qt-libdir],[AS_HELP_STRING([--with-qt-libdir=LIB_DIR],[specify qt lib path (overridden by pkgconfig)])], [qt_lib_path=$withval], [])
+  AC_ARG_WITH([qt-plugindir],[AS_HELP_STRING([--with-qt-plugindir=PLUGIN_DIR],[specify qt plugin path (overridden by pkgconfig)])], [qt_plugin_path=$withval], [])
+  AC_ARG_WITH([qt-bindir],[AS_HELP_STRING([--with-qt-bindir=BIN_DIR],[specify qt bin path])], [qt_bin_path=$withval], [])
 
   AC_ARG_WITH([qtdbus],
     [AS_HELP_STRING([--with-qtdbus],