Check for <sys/sdt.h> as well when determining to enable dtrace probes

Avoids auto-detecting dtrace is present on systems with the ISDN trace tool
named dtrace installed, but not the dynamic tracing facility named dtrace

configure.ac

@@ -66,6 +66,8 @@ AC_SYS_LARGEFILE
 XORG_PROG_RAWCPP
 
 dnl Check for dtrace program (needed to build Xserver dtrace probes)
+dnl Also checks for <sys/sdt.h>, since some Linux distros have an 
+dnl ISDN trace program named dtrace
 AC_ARG_WITH(dtrace, AS_HELP_STRING([--with-dtrace=PATH],
 	     [Enable dtrace probes (default: enabled if dtrace found)]),
 	     [WDTRACE=$withval], [WDTRACE=auto])
@@ -82,6 +84,11 @@ if test "x$WDTRACE" = "xyes" -o "x$WDTRACE" = "xauto" ; then
 			AC_MSG_FAILURE([dtrace requested but not found])
 		fi
 		WDTRACE="no"
+	else
+		AC_CHECK_HEADER(sys/sdt.h, [HAS_SDT_H="yes"], [HAS_SDT_H="no"])
+		if test "x$WDTRACE" = "xauto" -a "x$HAS_SDT_H" = "xno" ; then
+			WDTRACE="no"
+		fi
 	fi
 fi
 if test "x$WDTRACE" != "xno" ; then