use --libexecdir instead of ignoring it first and reinventing it later

Commit 7b93fadf3a "Add basic version
of bridge helper" put the bridge helper executable into a fixed
${prefix}/libexec/ location, instead of using ${libexecdir} for
this.  At the same time, --libexecdir is being happily ignored
by ./configure.  Even more, the same patch sets unused $libexecdir
variable in the generated config-host.mak, and uses fixed string
(\${prefix}/libexecdir) for the bridge helper binary.

Fix this braindamage by introducing $libexecdir variable, using
it for the bridge helper binary, and recognizing --libexecdir.

This patch is applicable to stable-1.1.

Reviewed-by: Andreas Färber <afaerber@suse.de>
Reviewed-by: Corey Bryant <coreyb@linux.vnet.ibm.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Cc: Corey Bryant <coreyb@linux.vnet.ibm.com>
Cc: Richa Marwaha <rmarwah@linux.vnet.ibm.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

configure

@@ -183,6 +183,7 @@ datadir="\${prefix}/share"
 qemu_docdir="\${prefix}/share/doc/qemu"
 bindir="\${prefix}/bin"
 libdir="\${prefix}/lib"
+libexecdir="\${prefix}/libexec"
 includedir="\${prefix}/include"
 sysconfdir="\${prefix}/etc"
 confsuffix="/qemu"
@@ -633,6 +634,8 @@ for opt do
   ;;
   --libdir=*) libdir="$optarg"
   ;;
+  --libexecdir=*) libexecdir="$optarg"
+  ;;
   --includedir=*) includedir="$optarg"
   ;;
   --datadir=*) datadir="$optarg"
@@ -643,7 +646,7 @@ for opt do
   ;;
   --sysconfdir=*) sysconfdir="$optarg"
   ;;
-  --sbindir=*|--libexecdir=*|--sharedstatedir=*|--localstatedir=*|\
+  --sbindir=*|--sharedstatedir=*|--localstatedir=*|\
   --oldincludedir=*|--datarootdir=*|--infodir=*|--localedir=*|\
   --htmldir=*|--dvidir=*|--pdfdir=*|--psdir=*)
     # These switches are silently ignored, for compatibility with
@@ -3085,6 +3088,7 @@ echo "Install prefix    $prefix"
 echo "BIOS directory    `eval echo $qemu_datadir`"
 echo "binary directory  `eval echo $bindir`"
 echo "library directory `eval echo $libdir`"
+echo "libexec directory `eval echo $libexecdir`"
 echo "include directory `eval echo $includedir`"
 echo "config directory  `eval echo $sysconfdir`"
 if test "$mingw32" = "no" ; then
@@ -3188,14 +3192,14 @@ echo all: >> $config_host_mak
 echo "prefix=$prefix" >> $config_host_mak
 echo "bindir=$bindir" >> $config_host_mak
 echo "libdir=$libdir" >> $config_host_mak
+echo "libexecdir=$libexecdir" >> $config_host_mak
 echo "includedir=$includedir" >> $config_host_mak
 echo "mandir=$mandir" >> $config_host_mak
 echo "sysconfdir=$sysconfdir" >> $config_host_mak
 echo "qemu_confdir=$qemu_confdir" >> $config_host_mak
 echo "qemu_datadir=$qemu_datadir" >> $config_host_mak
 echo "qemu_docdir=$qemu_docdir" >> $config_host_mak
-echo "libexecdir=\${prefix}/libexec" >> $config_host_mak
-echo "CONFIG_QEMU_HELPERDIR=\"$prefix/libexec\"" >> $config_host_mak
+echo "CONFIG_QEMU_HELPERDIR=\"$libexecdir\"" >> $config_host_mak
 
 echo "ARCH=$ARCH" >> $config_host_mak
 if test "$debug_tcg" = "yes" ; then