autotools: add pthreads linker support

This is needed for people that don't use any of the following:
JACK, ALSA, libmikmod, Shout.

configure.ac

@@ -148,6 +148,10 @@ dnl AC_CHECK_HEADER(langinfo.h,[enable_langinfo=yes;AC_DEFINE(HAVE_LANGINFO,1,[D
 AM_LANGINFO_CODESET
 AC_CHECK_HEADER(locale.h,[enable_locale=yes;AC_DEFINE(HAVE_LOCALE,1,[Define if locale.h is present])],enable_locale=no)
 
+ACX_PTHREAD([CC="$PTHREAD_CC"
+             LIBS="$PTHREAD_LIBS $LIBS"
+             CFLAGS="$PTHREAD_CFLAGS $CFLAGS"])
+
 if test x$enable_tcp = xno; then
 	# if we don't support TCP, we don't need IPv6 either
 	enable_ipv6=no