libsecureboot ensure correct BUILD_UTC

If using stat(1) on BUILD_UTC_FILE we should use -L incase
it is a symlink.

If we have new enough bmake though we can just use ${BUILD_UTC_FILE:mtime}

lib/libsecureboot/Makefile.inc

@@ -126,7 +126,11 @@ BUILD_UTC?= ${SOURCE_DATE_EPOCH}
 BUILD_UTC_FILE:= ${.PARSEDIR:tA}/${.PARSEFILE}
 .endif
 # you can of course set BUILD_UTC to any value you like
-BUILD_UTC?= ${${STAT:Ustat} -f %m ${BUILD_UTC_FILE}:L:sh}
+.if ${MAKE_VERSION} > 20230509
+BUILD_UTC?= ${BUILD_UTC_FILE:mtime}
+.else
+BUILD_UTC?= ${${STAT:Ustat} -L -f %m ${BUILD_UTC_FILE}:L:sh}
+.endif
 
 # Generate ta.h containing one or more PEM encoded trust anchors in ta_PEM.
 #