Require libexpat in ./configure

configure.ac

@@ -16,6 +16,7 @@ AC_PROG_CC
 AC_PROG_LN_S
 AM_PROG_CC_C_O
 AC_CHECK_LIB([m],[exp],,AC_MSG_ERROR([Libm missing]))
+AC_CHECK_LIB([expat],[XML_Parse],,AC_MSG_ERROR([Libexpat missing]))
 PKG_CHECK_MODULES([CHECK], [check >= 0.9.12],[have_check="yes"],AC_MSG_WARN([Check not found or check version lower than 0.9.12; cannot run unit tests!])
 [have_check="no"])
 AM_CONDITIONAL([HAVE_CHECK], [test x"$have_check" = "xyes"])