Do not build pn532_spi if spi is not available

Check if the include file linux/spi/spidev.h is available and, only if
the header file is present, add the driver pn532_spi for "default" and
"all" driver configuration.

This Linux header file is not present on Mac OS X for example.

configure.ac

@@ -48,6 +48,7 @@ fi
 AC_HEADER_STDC
 AC_HEADER_STDBOOL
 AC_CHECK_HEADERS([fcntl.h limits.h stdio.h stdlib.h stdint.h stddef.h stdbool.h sys/ioctl.h sys/param.h sys/time.h termios.h])
+AC_CHECK_HEADERS([linux/spi/spidev.h], [spi_available="yes"])
 AC_CHECK_FUNCS([memmove memset select strdup strerror strstr strtol usleep],
 	       [AC_DEFINE([_XOPEN_SOURCE], [600], [Enable POSIX extensions if present])])
 

m4/libnfc_drivers.m4

@@ -25,10 +25,18 @@ AC_DEFUN([LIBNFC_ARG_WITH_DRIVERS],
 
   case "${DRIVER_BUILD_LIST}" in
     default)
-                  DRIVER_BUILD_LIST="acr122_usb acr122s arygon pn53x_usb pn532_uart pn532_spi"
+                  DRIVER_BUILD_LIST="acr122_usb acr122s arygon pn53x_usb pn532_uart"
+                  if test x"$spi_available" = x"yes"
+                  then
+                      DRIVER_BUILD_LIST="$DRIVER_BUILD_LIST pn532_spi"
+                  fi
                   ;;
     all)
-                  DRIVER_BUILD_LIST="acr122_pcsc acr122_usb acr122s arygon pn53x_usb pn532_uart pn532_spi"
+                  DRIVER_BUILD_LIST="acr122_pcsc acr122_usb acr122s arygon pn53x_usb pn532_uart"
+                  if test x"$spi_available" = x"yes"
+                  then
+                      DRIVER_BUILD_LIST="$DRIVER_BUILD_LIST pn532_spi"
+                  fi
                   ;;
   esac