st/xvmc: fix library installation dir

Fixes https://bugs.freedesktop.org/show_bug.cgi?id=47878

v2: some reordering and while at it also fix the
    comment in Makefile.xvmc

Signed-off-by: Christian König <deathsimple@vodafone.de>
diff --git a/configure.ac b/configure.ac
index 135bacf..21e4308 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1816,6 +1816,14 @@
     MESA_LLVM=0
 fi
 
+dnl Directory for XVMC libs
+AC_ARG_WITH([xvmc-libdir],
+    [AS_HELP_STRING([--with-xvmc-libdir=DIR],
+        [directory for the XVMC libraries @<:@default=${libdir}@:>@])],
+    [XVMC_LIB_INSTALL_DIR="$withval"],
+    [XVMC_LIB_INSTALL_DIR='${libdir}'])
+AC_SUBST([XVMC_LIB_INSTALL_DIR])
+
 dnl Directory for VDPAU libs
 AC_ARG_WITH([vdpau-libdir],
     [AS_HELP_STRING([--with-vdpau-libdir=DIR],