Bugfix: Check for libudev header (not just library) in configure, and document optional dependency

README

@@ -44,6 +44,8 @@ Dependencies:
 	(This sdk is mandatory for GPU mining)
 	AMD ADL SDK		http://developer.amd.com/sdks/ADLSDK
 	(This sdk is mandatory for ATI GPU monitoring & clocking)
+	libudev headers
+	(This is only required for FPGA auto-detection)
 
 CGMiner specific configuration options:
 	--enable-cpumining      Build with cpu mining support(default disabled)

configure.ac

@@ -266,7 +266,7 @@ if test "x$bitforce" != xno; then
 		[libudev=auto]
 		)
 	if test "x$libudev" != "xno"; then
-		AC_CHECK_LIB([udev], [udev_device_get_devnode], [
+		AC_CHECK_HEADER([libudev.h],[
 			libudev=yes
 			UDEV_LIBS=-ludev
 			AC_DEFINE([HAVE_LIBUDEV], [1], [Defined to 1 if libudev is wanted])