Fedora 28: Fix "Macro %_dracutdir has empty body"

If you run ./configure --with-config=srpm, it will not trigger
the user m4 scripts to populate the dracut and udev directories.
This causes a build error on Fedora 28.  Make the dracut and
udev lines conditional to get around this.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Closes #7326
Closes #7328

config/zfs-build.m4

@@ -185,10 +185,23 @@ AC_DEFUN([ZFS_AC_RPM], [
 	RPM_DEFINE_COMMON+=' --define "$(DEBUGINFO_ZFS) 1"'
 	RPM_DEFINE_COMMON+=' --define "$(ASAN_ZFS) 1"'
 
-	RPM_DEFINE_UTIL='--define "_dracutdir $(dracutdir)"'
-	RPM_DEFINE_UTIL+=' --define "_udevdir $(udevdir)"'
-	RPM_DEFINE_UTIL+=' --define "_udevruledir $(udevruledir)"'
-	RPM_DEFINE_UTIL+=' --define "_initconfdir $(DEFAULT_INITCONF_DIR)"'
+
+	RPM_DEFINE_UTIL=' --define "_initconfdir $(DEFAULT_INITCONF_DIR)"'
+
+	dnl # Make the next three RPM_DEFINE_UTIL additions conditional, since
+	dnl # their values may not be set when running:
+	dnl #
+	dnl #	./configure --with-config=srpm
+	dnl #
+	AS_IF([test -n "$dracutdir" ], [
+		RPM_DEFINE_UTIL='--define "_dracutdir $(dracutdir)"'
+	])
+	AS_IF([test -n "$udevdir" ], [
+		RPM_DEFINE_UTIL+=' --define "_udevdir $(udevdir)"'
+	])
+	AS_IF([test -n "$udevruledir" ], [
+		RPM_DEFINE_UTIL+=' --define "_udevdir $(udevruledir)"'
+	])
 	RPM_DEFINE_UTIL+=' $(DEFINE_INITRAMFS)'
 	RPM_DEFINE_UTIL+=' $(DEFINE_SYSTEMD)'