new configure script detects them and now this code acts according to what
API that was detected

1. Try with _THREAD_SAFE instead of _REENTRANT, as AIX seems to require it
   and if _REENTRANT is required we should already have it set since one of
   the previous tests.
2. Added API-detection for what kind of strerror_r() that is provided. The
   POSIX style or the glibc style.

Tor Arntsen provided the necessary feedback these changes are based upon.

'maketgz'

while perl often is installed in /usr/local/bin or elsewhere

Added a more verbose comment about what strerror_r() can set errno to in
case of failure.
This file still doesn't build on Solaris due to a missing 'sys_nerr' symbol.

Curl_strerror() that attempts to be thread-safe _and_ works on Windows too!

chdir to absolute directory names instead. (this flaw exists in the shell
version too)

portability. I put it in this directory instead of the root since I think
perhaps it makes more sense.

if large file support is available

when converting to ints