Use AS_IF rather than plain shell "if" in pthread-check.

 Use AS_IF rather than plain shell "if" in pthread-check.
 
 Autoconf generates additional code for the first AC_CHECK_HEADERS call in
 the script. If the first call is within an if-block, the additional code is
 put inside the if-block too, even though it is needed by subsequent
 AC_CHECK_HEADERS checks and should always be executed. When I moved the
 pthread-related checks earlier in the script, the pthread.h test inside
 the block became the very first AC_CHECK_HEADERS call in the script,
 triggering that problem.
 
 To fix, use AS_IF instead of plain shell if. AS_IF knows about that issue,
 and makes sure the additional code is always executed. To be completely
 safe from this issue (and others), we should always be using AS_IF instead
 of plain if, but that seems like excessive caution given that this is the
 first time we have trouble like this. Plain if-then is more readable than
 AS_IF.
 
 This should fix compilation with --disable-thread-safety, and hopefully the
 buildfarm failure on forgmouth, related to mingw standard headers, too.
 I backpatched the previous fixes to 9.5, but it's starting to look like
 these changes are too fiddly to backpatch, so commit this to master only,
 and revert all the pthread-related configure changes in 9.5.