In the alpha3 release tarball, php_xdebug.h contains some git merge detritus that prevents it from building at all:

<<<<<<< Updated upstream
#define XDEBUG_VERSION    "3.3.0alpha3-dev"
=======
#define XDEBUG_VERSION    "3.3.0alpha3"
>>>>>>> Stashed changes

Trying to build this will give you:

In file included from /rpmbuild/BUILD/xdebug-3.3.0alpha3/xdebug.c:47:0:
/rpmbuild/BUILD/xdebug-3.3.0alpha3/php_xdebug.h:21:1: error: version control conflict marker in file
 <<<<<<< Updated upstream
 ^~~~~~~
/rpmbuild/BUILD/xdebug-3.3.0alpha3/php_xdebug.h:24:0: warning: "XDEBUG_VERSION" redefined
 #define XDEBUG_VERSION    "3.3.0alpha3"

/rpmbuild/BUILD/xdebug-3.3.0alpha3/php_xdebug.h:22:0: note: this is the location of the previous definition
 #define XDEBUG_VERSION    "3.3.0alpha3-dev"

In file included from /rpmbuild/BUILD/xdebug-3.3.0alpha3/src/lib/hash.h:22:0,
                 from /rpmbuild/BUILD/xdebug-3.3.0alpha3/src/base/base_globals.h:20,
                 from /rpmbuild/BUILD/xdebug-3.3.0alpha3/php_xdebug.h:38,
                 from /rpmbuild/BUILD/xdebug-3.3.0alpha3/xdebug.c:47:
/rpmbuild/BUILD/xdebug-3.3.0alpha3/src/lib/llist.h:35:2: error: unknown type name 'xdebug_llist_dtor'
  xdebug_llist_dtor dtor;
  ^~~~~~~~~~~~~~~~~
/rpmbuild/BUILD/xdebug-3.3.0alpha3/src/lib/llist.h:40:34: error: unknown type name 'xdebug_llist_dtor'
 xdebug_llist *xdebug_llist_alloc(xdebug_llist_dtor dtor);
                                  ^~~~~~~~~~~~~~~~~
/rpmbuild/BUILD/xdebug-3.3.0alpha3/src/lib/llist.h:41:41: error: unknown type name 'xdebug_llist_dtor'
 void xdebug_llist_init(xdebug_llist *l, xdebug_llist_dtor dtor);
                                         ^~~~~~~~~~~~~~~~~
make: *** [Makefile:261: xdebug.lo] Error 1

(The latter messages I'm guessing are because of how the header got merged by the merge conflict in the source, so I haven't looked into them deeply yet.)