./configure fails if libpthread is absent

[RossBencina]

(thanks to simphone project for reporting this by email)

some (newer) linux systems have pthread_create in libc; libpthread may or may not be present as a stub/wrapper library. if libpthread is not present, portaudio configure would fail for no good reason. A workaround is:

diff -urb portaudio/configure portaudio/configure
--- portaudio/configure
+++ portaudio/configure
@@ -16169,8 +16170,6 @@
 $as_echo "$ac_cv_lib_pthread_pthread_create" >&6; }
 if test "x$ac_cv_lib_pthread_pthread_create" = xyes; then :
   have_pthread="yes"
-else
-  as_fn_error $? "libpthread not found!" "$LINENO" 5
 fi
 
 
diff -urb portaudio/configure.in portaudio/configure.in
--- portaudio/configure.in
+++ portaudio/configure.in
@@ -390,8 +391,7 @@
 
         CFLAGS="$CFLAGS -I\$(top_srcdir)/src/os/unix"
 
-        AC_CHECK_LIB(pthread, pthread_create,[have_pthread="yes"],
-                AC_MSG_ERROR([libpthread not found!]))
+        AC_CHECK_LIB(pthread, pthread_create,[have_pthread="yes"])
 
         if [[ "$have_alsa" = "yes" ] && [ "$with_alsa" != "no" ]] ; then
            DLL_LIBS="$DLL_LIBS -lasound"

of course, simply removing the error check is not the best solution here; one should probably check for pthread_create
also in libc, and then fail if not found there either. but we leave such improvements to the portaudio team. for us, simply not failing is good enough, as we've not seen unix systems that lack pthread support for a
very long time now.

[RossBencina]

So I guess the correct fix should be:

-        AC_CHECK_LIB(pthread, pthread_create,[have_pthread="yes"],
-                AC_MSG_ERROR([libpthread not found!]))
+        AC_CHECK_LIB(pthread, pthread_create,[have_libpthread="yes"])
+        AC_CHECK_LIB(c, pthread_create,[have_libc_pthread="yes"])
+
+         if [[ "$have_libpthread" = "no" ] && [ "$have_libc_pthread" != "no" ]] ; then
+            AC_MSG_ERROR([pthread not found!]))
+         fi

maybe also omit -lpthread when `$have_libpthread$=="no"