Include <sys/select.h> for select() if available.

According to the man page, <sys/select.h> is the header to include
on POSIX systems for select() and associated APIs.
Fixes compilation with some versions of Cygwin (Github issue #49).

configure.ac:
runtime/mercury_conf.h.in
    Define MR_HAVE_SYS_SELECT_H if <sys/select.h> is available.

runtime/mercury_reg_workarounds.c
runtime/mercury_reg_workarounds.h
    Include <sys/select.h> if MR_HAVE_SYS_SELECT_H is defined.

configure.ac

@@ -1420,7 +1420,7 @@ MERCURY_CHECK_FOR_HEADERS( \
         sys/types.h sys/stat.h fcntl.h termios.h sys/ioctl.h \
         sys/resource.h sys/stropts.h windows.h dirent.h getopt.h malloc.h \
         semaphore.h pthread.h time.h spawn.h fenv.h sys/mman.h sys/sem.h \
-        sched.h utmpx.h dispatch/dispatch.h stdint.h)
+        sched.h utmpx.h dispatch/dispatch.h stdint.h sys/select.h )
 
 if test "$MR_HAVE_GETOPT_H" = 1; then
     GETOPT_H_AVAILABLE=yes
@@ -2426,6 +2426,9 @@ AC_MSG_CHECKING(to see if we can handle contexts blocking on IO)
 AC_CACHE_VAL(mercury_cv_can_do_pending_io,
     AC_TRY_RUN(
         [
+            #ifdef MR_HAVE_SYS_SELECT_H
+            #include <sys/select.h>
+            #endif
             #include <sys/types.h>
             #include <sys/time.h>
             int main() {

runtime/mercury_conf.h.in

@@ -140,7 +140,8 @@
 //  MR_HAVE_UTMPX_H         we have <utmpx.h>
 //  MR_HAVE_SYS_RESOURCE_H  we have <sys/resource.h>
 //  MR_HAVE_DISPATCH_DISPATCH_H we have <dispatch/dispatch.h>
-//  MR_HAVE_STDINT          we have <stdint.h>
+//  MR_HAVE_STDINT_H        we have <stdint.h>
+//  MR_HAVE_SYS_SELECT_H    we have <sys/select.h>
 
 #undef  MR_HAVE_SYS_SIGINFO_H
 #undef  MR_HAVE_SYS_SIGNAL_H
@@ -175,6 +176,7 @@
 #undef  MR_HAVE_SYS_RESOURCE_H
 #undef  MR_HAVE_DISPATCH_DISPATCH_H
 #undef  MR_HAVE_STDINT_H
+#undef  MR_HAVE_SYS_SELECT_H
 
 // MR_HAVE_POSIX_TIMES is defined if we have the POSIX
 // `struct tms' struct and times() function.

runtime/mercury_reg_workarounds.c

@@ -14,15 +14,19 @@
 
 #ifdef  MR_CAN_DO_PENDING_IO
 
-  #ifdef MR_BZERO_NEEDS_STRINGS_HEADER
-    #include <strings.h>
-  #endif
+#ifdef MR_BZERO_NEEDS_STRINGS_HEADER
+  #include <strings.h>
+#endif
 
-#include <sys/types.h>  // for fd_set and FD_ZERO()
-#include <sys/time.h>   // for FD_ZERO()
+#ifdef MR_HAVE_SYS_SELECT_H
+  #include <sys/select.h>   // for select() -- POSIX
+#endif
+
+#include <sys/types.h>      // for fd_set and FD_ZERO()
+#include <sys/time.h>       // for FD_ZERO()
 
 #ifdef MR_HAVE_UNISTD_H
-  #include <unistd.h>   // for FD_ZERO()
+  #include <unistd.h>       // for FD_ZERO()
 #endif
 
 void

runtime/mercury_reg_workarounds.h

@@ -12,6 +12,9 @@
 #include "mercury_conf.h"
 
 #ifdef MR_CAN_DO_PENDING_IO
+  #ifdef MR_HAVE_SYS_SELECT_H
+    #include <sys/select.h>     // for select() -- POSIX
+  #endif
   #include <sys/types.h>        // for fd_set
   #include <sys/time.h>         // for FD_ZERO()
 #endif