Configure: fix some checks about Qt5Core

Pointed by BtbN on IRC

configure.ac

@@ -3689,7 +3689,7 @@ AC_ARG_ENABLE(qt, [
 ])
 AS_IF([test "${enable_qt}" != "no"], [
   PKG_CHECK_MODULES([QT], [Qt5Core >= 5.1.0 Qt5Widgets Qt5Gui], [
-      QT_PATH="$(eval $PKG_CONFIG --variable=exec_prefix QtCore)"
+      QT_PATH="$(eval $PKG_CONFIG --variable=exec_prefix Qt5Core)"
       AC_PATH_PROGS(MOC, [moc-qt5 moc], moc, [${QT_PATH}/bin ${CONTRIB_DIR}/bin])
       AC_PATH_PROG(RCC, [rcc-qt5 rcc], rcc, [${QT_PATH}/bin ${CONTRIB_DIR}/bin])
       AC_PATH_PROGS(UIC, [uic-qt5 uic], uic, [${QT_PATH}/bin ${CONTRIB_DIR}/bin])