configure: fix static link with popt

Use pkg-config to find the popt library.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>

---
Patch applied upstream:
    https://github.com/mikebrady/shairport-sync/commit/1f759e57e8c0682613eaaf89a46fdf6db4c6b5cd

diff -durN shairport-sync-2.1.8.orig/configure.ac shairport-sync-2.1.8/configure.ac
--- shairport-sync-2.1.8.orig/configure.ac	2014-11-29 14:14:36.655955733 +0100
+++ shairport-sync-2.1.8/configure.ac	2014-11-29 14:16:21.072719327 +0100
@@ -31,7 +31,10 @@
 AC_CHECK_LIB([daemon],[daemon_log], , AC_MSG_ERROR(libdaemon needed))
 AC_CHECK_LIB([pthread],[pthread_create], , AC_MSG_ERROR(pthread library needed))
 AC_CHECK_LIB([m],[exp], , AC_MSG_ERROR(maths library needed))
-AC_CHECK_LIB([popt],[poptGetContext], , AC_MSG_ERROR(libpopt needed))
+PKG_CHECK_MODULES(
+    [POPT], [popt],
+    [LIBS="${POPT_LIBS} ${LIBS}"
+     AC_DEFINE([HAVE_LIBPOPT],[1],[Define to 1 if you have popt])])
 
 # Look for piddir flag
 AC_ARG_WITH(piddir, [ --with-piddir=<pathname> Specify a pathname to a directory in which to write the PID file.], [