Fix configure for users that have 'WITHOUT_GNU_GREP_COMPAT=yes' set
m4/qt.m4 fixed upstream in git 211844f.

--- configure.orig	2016-12-20 07:27:31 UTC
+++ configure
@@ -18746,7 +18746,7 @@ fi
     { $as_echo "$as_me:${as_lineno-$LINENO}: checking moc version" >&5
 $as_echo_n "checking moc version... " >&6; }
     mocversion=`$MOC -v 2>&1`
-    mocversiongrep=`echo $mocversion | grep "Qt 5\|moc 5"`
+    mocversiongrep=`echo $mocversion | grep -E "Qt 5|moc 5"`
     if test x"$mocversiongrep" != x"$mocversion"; then
       { $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
 $as_echo "no" >&6; }
@@ -18844,7 +18844,7 @@ else
 fi
 
       mocversion=`$MOC2 -v 2>&1`
-      mocversiongrep=`echo $mocversion | grep "Qt 5\|moc-qt5 5\|moc 5"`
+      mocversiongrep=`echo $mocversion | grep -E "Qt 5|moc-qt5 5|moc 5"`
       if test x"$mocversiongrep" != x"$mocversion"; then
         if test -n "$ac_tool_prefix"; then
   # Extract the first word of "${ac_tool_prefix}qtchooser", so it can be a program name with args.
@@ -18940,7 +18940,7 @@ fi
 
         qt5tooldir=`QT_SELECT=qt5 qtchooser -print-env | grep QTTOOLDIR | cut -d '=' -f 2 | cut -d \" -f 2`
         mocversion=`$qt5tooldir/moc -v 2>&1`
-        mocversiongrep=`echo $mocversion | grep "Qt 5\|moc 5"`
+        mocversiongrep=`echo $mocversion | grep -E "Qt 5|moc 5"`
         if test x"$mocversiongrep" != x"$mocversion"; then
           # no valid moc found
           have_qt5_libs="no";