Make libudev optional.

configure.ac

@@ -86,10 +86,15 @@ SP_PKGLIBS=""
 
 case $target_os in
 *linux*)
-	# On Linux libudev is currently a hard requirement.
+	# On Linux libudev is currently required for enumeration.
+	AC_ARG_WITH([libudev],
+            [AS_HELP_STRING([--with-libudev],
+            [use libudev for serial port enumeration @<:@default=check@:>@])],
+            [], [with_libudev=check])
+	AS_IF([test "x$with_libudev" != xno], [
 	PKG_CHECK_MODULES([libudev], [libudev >= 0],
 		[CFLAGS="$CFLAGS $libudev_CFLAGS"; LIBS="$LIBS $libudev_LIBS";
-		SP_PKGLIBS="$SP_PKGLIBS libudev"])
+		SP_PKGLIBS="$SP_PKGLIBS libudev"; AC_DEFINE(HAVE_LIBUDEV)])], [])
 	;;
 *darwin*)
 	LDFLAGS="$LDFLAGS -Wl,-framework -Wl,IOKit -Wl,-framework -Wl,CoreFoundation"

serialport.c

@@ -47,7 +47,9 @@
 #include <sys/syslimits.h>
 #endif
 #ifdef __linux__
+#ifdef HAVE_LIBUDEV
 #include "libudev.h"
+#endif
 #include "linux/serial.h"
 #include "linux_termios.h"
 
@@ -460,7 +462,7 @@ out_release:
 	IOObjectRelease(iter);
 out_done:
 #endif
-#ifdef __linux__
+#if defined(__linux__) && defined(HAVE_LIBUDEV)
 	struct udev *ud;
 	struct udev_enumerate *ud_enumerate;
 	struct udev_list_entry *ud_list;