by setting _THREAD_SAFE (and -qthreaded) before strerror_r() is checked for.

the -qthreaded compiler option

land with many warnings and even compiler errors due to missing structs etc

decent quote about the define, taken from
http://www.opengroup.org/onlinepubs/007908799/xsh/compilation.html

and if so, define _XOPEN_SOURCE to 500 in an attempt to build with less
warnings (on the 64bit versions)

possibly only on some platforms, but it happens on my Solaris 2.7 box and I
don't know anyone else that regularly build curl with krb4 support.

versions

render a build impossible

disabled SSL with --without-ssl. This previously made the Makefiles use
the SSL libs even though told not to.

modified the compiler and link options so nothing can be tested for after
this check, as the c-ares lib might not have been built yet!

directory. The provided ares dir is probably up-to-date, but more importantly
it is often not built yet at the time when this configure script runs.

that we use a library that is recent enough to build with the latest libcurl.

works. No code actually uses the HAVE_NI_WITHSCOPEID (that a positive test
results in), but this is still only for testing purposes.

a check for there *_r-versions too

libs we want. $libsuff is the magic variable that contains a suffix (which
might be blank). Tor Arntsen brought details and verified this fix.

CURL_CC_DEBUG_OPTS function in acinclude.m4