configure: Parse -device-option value correctly

The regular expression does not parse correctly when a device option value
contains the character '=' (e.g. QMAKE_CFLAGS="-D_FILE_OFFSET_BITS=64").

In order to break string at the first equal sign and to simplify code,
use "cut" command as in other places in configure script.

Task-number: QTBUG-33584
Change-Id: I05b474d2ba6bff84c1e40d00475963bab36d94b6
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>

configure

@@ -1609,8 +1609,8 @@ while [ "$#" -gt 0 ]; do
         [ "$XPLATFORM" = "undefined" ] && exit 101
         ;;
     device-option)
-        DEV_VAR=`echo $VAL | sed "s,^\(.*\)=.*,\1,"`
-        DEV_VAL=`echo $VAL | sed "s,^.*=\(.*\),\1,"`
+        DEV_VAR=`echo $VAL | cut -d '=' -f 1`
+        DEV_VAL=`echo $VAL | cut -d '=' -f 2-`
         DeviceVar set $DEV_VAR "$DEV_VAL"
         ;;
     qpa)