Compile error in rgw/rgw_xml.h in 0.46

[Guido Winkelmann <guido-ceph@thisisnotatest.de>]

Compiling Ceph 0.46 fails at rgw/rgw_dencoder.cc with the following errors:

In file included from rgw/rgw_dencoder.cc:7:
rgw/rgw_acl_s3.h:9:19: error: expat.h: No such file or directory
In file included from rgw/rgw_acl_s3.h:11,
                 from rgw/rgw_dencoder.cc:7:
rgw/rgw_xml.h:59: error: 'XML_Parser' does not name a type
make[3]: *** [ceph_dencoder-rgw_dencoder.o] Error 1
make[3]: Leaving directory `/root/ceph-0.46/src'
make[2]: *** [all-recursive] Error 1
make[2]: Leaving directory `/root/ceph-0.46/src'
make[1]: *** [all] Error 2
make[1]: Leaving directory `/root/ceph-0.46/src'
make: *** [all-recursive] Error 1

This is on CentOS 6.2.

I managed to get it to compile by installing expat-devel first. Maybe the 
configure script should check for the existence of the expat header files?

	Guido