patchconfigure.ac: fix pthreads detection on Mac OS X

> The configure script checks whether certain flags are required to use
> pthreads. But it did not consider that *none* might be needed (as is the
> case on Mac OS X). This lead to configure adding "-mt" to the list of
> flags (which does nothing on OS X except producing a warning). This in
> turn triggered a compiler warning on every single file.
>
> To solve this, we now first check if pthreads work without extra flags.
> This means the check is now order dependant, hence a comment is added
> explaining this, and the reasons for it.
>
> Note that it might be possible to write an order independent test, but
> it does not seem worth the extra effort required for implementing and
> testing such a solution, when this simple solution exists and works.
>
> Signed-off-by: Max Horn <max@quendi.de>
> ---
>
> This is actually a revised version from my patch
>  "Change configure to check if pthreads are usable without any extra flags"
> from July. I simply had forgotten all about it :-(.

> Chers,
> Max
>
>  configure.ac | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
>
> diff --git a/configure.ac b/configure.ac
> index ad215cc..41ac9a5 100644
> --- a/configure.ac
> +++ b/configure.ac
> @@ -1021,7 +1021,17 @@ if test -n "$USER_NOPTHREAD"; then
>  # -D_REENTRANT' or some such.
>  elif test -z "$PTHREAD_CFLAGS"; then
>    threads_found=no
> -  for opt in -mt -pthread -lpthread; do
> +  # Attempt to compile and link some code using pthreads to determine
> +  # required linker flags. The order is somewhat important here: We
> +  # first try it without any extra flags, to catch systems where
> +  # pthreads are part of the C library, then go on testing various other
> +  # flags. We do so to avoid false positives. For example, on Mac OS X
> +  # pthreads are part of the C library; moreover, the compiler allows us
> +  # to add "-mt" to the CFLAGS (although it will do nothing except
> +  # trigger a warning about an unused flag). Hence if we checked for
> +  # "-mt" before "" we would end up picking it. But unfortunately this
> +  # would then trigger compiler warnings on every single file we compile.
> +  for opt in "" -mt -pthread -lpthread; do
>       old_CFLAGS="$CFLAGS"
>       CFLAGS="$opt $CFLAGS"
>       AC_MSG_CHECKING([for POSIX Threads with '$opt'])

> Max Horn <max@quendi.de> writes:
> 
>> The configure script checks whether certain flags are required to use
>> pthreads. But it did not consider that *none* might be needed (as is the
>> case on Mac OS X). This lead to configure adding "-mt" to the list of
>> flags (which does nothing on OS X except producing a warning). This in
>> turn triggered a compiler warning on every single file.
>> 
>> To solve this, we now first check if pthreads work without extra flags.
>> This means the check is now order dependant, hence a comment is added
>> explaining this, and the reasons for it.
>> 
>> Note that it might be possible to write an order independent test, but
>> it does not seem worth the extra effort required for implementing and
>> testing such a solution, when this simple solution exists and works.
>> 
>> Signed-off-by: Max Horn <max@quendi.de>
>> ---
>> 
>> This is actually a revised version from my patch
>> "Change configure to check if pthreads are usable without any extra flags"
>> from July. I simply had forgotten all about it :-(.
> 
> Will queue,

> but we would need wider testing to avoid "compiles well
> without an option but fails to link" issues similar to cea13a8
> (Improve test for pthreads flag, 2011-03-28) on other people's
> platforms (I know you tested on Mac OS X and over there it compiles
> and links well---I am worried about others).