Fails to build from source on PPC64LE

Description:
------------
xhprof fails to build from source on ppc64le.
This has been tested on Debian Jessie.
Version is master 0bbf2a2 from https://github.com/phacility/xhprof.git



Test script:
---------------
# make

Actual result:
--------------
/bin/bash /home/xhprof/upstream/xhprof/extension/libtool --mode=compile cc -I. -I/home/xhprof/upstream/xhprof/extension -DPHP_ATOM_INC -I/home/xhprof/upstream/xhprof/extension/include -I/home/xhprof/upstream/xhprof/extension/main -I/home/xhprof/upstream/xhprof/extension -I/usr/include/php5 -I/usr/include/php5/main -I/usr/include/php5/TSRM -I/usr/include/php5/Zend -I/usr/include/php5/ext -I/usr/include/php5/ext/date/lib -DHAVE_CONFIG_H -g -O2 -c /home/xhprof/upstream/xhprof/extension/xhprof.c -o xhprof.lo
libtool: compile: cc -I. -I/home/xhprof/upstream/xhprof/extension -DPHP_ATOM_INC -I/home/xhprof/upstream/xhprof/extension/include -I/home/xhprof/upstream/xhprof/extension/main -I/home/xhprof/upstream/xhprof/extension -I/usr/include/php5 -I/usr/include/php5/main -I/usr/include/php5/TSRM -I/usr/include/php5/Zend -I/usr/include/php5/ext -I/usr/include/php5/ext/date/lib -DHAVE_CONFIG_H -g -O2 -c /home/xhprof/upstream/xhprof/extension/xhprof.c -fPIC -DPIC -o .libs/xhprof.o
In file included from /usr/include/php5/main/php.h:34:0,
from /home/xhprof/upstream/xhprof/extension/xhprof.c:27:
/usr/include/php5/main/php_config.h:2481:0: warning: "_GNU_SOURCE" redefined
#define _GNU_SOURCE 1
^
/home/xhprof/upstream/xhprof/extension/xhprof.c:24:0: note: this is the location of the previous definition
# define _GNU_SOURCE
^
/home/xhprof/upstream/xhprof/extension/xhprof.c: In function 'hp_mode_hier_beginfn_cb':
/home/xhprof/upstream/xhprof/extension/xhprof.c:1235:3: error: impossible register constraint in 'asm'
asm volatile("rdtsc" : "=a" (__a), "=d" (__d));
^
Makefile:180: recipe for target 'xhprof.lo' failed
make: *** [xhprof.lo] Error 1