configure: Fix CONFIG_QEMU_HELPERDIR generation

We need to evaluate $libexecdir in configure, otherwise we literally end
up with "${prefix}/libexec" instead of the absolute path as
CONFIG_QEMU_HELPERDIR.
Signed-off-by: NJan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>

configure

@@ -3200,7 +3200,7 @@ echo "qemu_confdir=$qemu_confdir" >> $config_host_mak
 echo "qemu_datadir=$qemu_datadir" >> $config_host_mak
 echo "qemu_docdir=$qemu_docdir" >> $config_host_mak
 echo "qemu_localstatedir=$local_statedir" >> $config_host_mak
-echo "CONFIG_QEMU_HELPERDIR=\"$libexecdir\"" >> $config_host_mak
+echo "CONFIG_QEMU_HELPERDIR=\"`eval echo $libexecdir`\"" >> $config_host_mak
 
 echo "ARCH=$ARCH" >> $config_host_mak
 if test "$debug_tcg" = "yes" ; then