pthread: Disable support if pthread_t is *not* an integer type

The sanei_thread implementation assumes an integer type in case of
pthread based thread support.  As anything else is unlikely to work
correctly, it's safer to just fall back to forked processes.

acinclude.m4

@@ -235,6 +235,24 @@ AC_DEFUN([SANE_CHECK_PTHREAD],
     ],[ have_pthread=no; use_pthread=no ])
   fi
 
+  # Based on a similar test for pthread_key_t from the Python project.
+  # See https://bugs.python.org/review/25658/patch/19209/75870
+  AC_MSG_CHECKING(whether pthread_t is integer)
+  AC_COMPILE_IFELSE(
+    [AC_LANG_PROGRAM([[#include <pthread.h>]], [[pthread_t k; k * 1;]])],
+    [ac_pthread_t_is_integer=yes],
+    [ac_pthread_t_is_integer=no]
+  )
+  AC_MSG_RESULT($ac_pthread_t_is_integer)
+  if test "$ac_pthread_t_is_integer" = yes ; then
+    AC_DEFINE(PTHREAD_T_IS_INTEGER, 1,
+              [Define if pthread_t is integer.])
+  else
+    # Until the sanei_thread implementation is fixed.
+    have_pthread=no
+    use_pthread=no
+  fi
+
   if test $use_pthread = yes ; then
     AC_DEFINE_UNQUOTED(USE_PTHREAD, "$use_pthread",
                    [Define if pthreads should be used instead of forked processes.])