autotools: check for expat.h if pkg-config failed

Some distros provides libexpat without expat.pc. So PKG_CHECK_MODULES
fails even if libexpat is installed. To fix this AC_CHECK_HEADER added
back.

configure.ac

@@ -31,7 +31,8 @@ if test "x$with_libxml2" != xno; then
     CFLAGS=$old_CFLAGS
   fi
 else
-  PKG_CHECK_MODULES([expat], [expat >= 2.0.0], [], [AC_MSG_ERROR([expat not found; expat required.])])
+  PKG_CHECK_MODULES([expat], [expat >= 2.0.0], [],
+                    [AC_CHECK_HEADER(expat.h, [expat_LIBS="-lexpat"], [AC_MSG_ERROR([expat not found; expat required.])])])
 fi
 
 if test "x$with_libxml2" = xyes; then