Fix usage of strerror_r

[Michael Terry]

During build, the following warning appears:

page.c: In function 'stringErrorReport':
page.c:46:2: warning: return makes pointer from integer without a cast [enabled by default]
  return strerror_r(errno,(char *)err_message,128);
  ^

It indicates the XSI version of strerror_r is being used, but the code is expecting the GNU one (see the man page for strerror_r).

This bug will manifest as a lack of detailed error messages (because it will think strerror is returning NULL most of the time). But on the off-chance that 128 bytes isn't long enough for the error message, it means that electric-fence will try to print from a bogus address like 32 (errno value).