Cache _MPATH and pass it down into the modules build. Some NFS setups

make the find it does extremely expensive, so compute it only
once. Also make sure the 'traditional' module building method works at
the expense of a bit of duplicated code.

sys/conf/kern.pre.mk

@@ -198,6 +198,12 @@ SYSTEM_LD_TAIL= @${OBJCOPY} --strip-symbol gcc2_compiled. ${.TARGET} ; \
 	${SIZE} ${.TARGET} ; chmod 755 ${.TARGET}
 SYSTEM_DEP+= ${LDSCRIPT}
 
+# Calculate path for .m files early, if needed.
+.if !defined(_MPATH)
+__MPATH!=find ${S:tA}/ -name \*_if.m
+_MPATH=${__MPATH:H:O:u}
+.endif
+
 # MKMODULESENV is set here so that port makefiles can augment
 # them.
 
@@ -214,6 +220,7 @@ MKMODULESENV+=	MODULES_OVERRIDE="${MODULES_OVERRIDE}"
 .if defined(DEBUG)
 MKMODULESENV+=	DEBUG_FLAGS="${DEBUG}"
 .endif
+MKMODULESENV+=	_MPATH="${_MPATH}"
 
 # Detect kernel config options that force stack frames to be turned on.
 DDB_ENABLED!=	grep DDB opt_ddb.h || true ; echo

sys/conf/kmod.mk

@@ -355,8 +355,10 @@ vnode_if_typedef.h:
 .endif
 
 # Build _if.[ch] from _if.m, and clean them when we're done.
+.if !defined(_MPATH)
 __MPATH!=find ${SYSDIR:tA}/ -name \*_if.m
 _MPATH=${__MPATH:H:O:u}
+.endif
 .PATH.m: ${_MPATH}
 .for _s in ${SRCS:M*_if.[ch]}
 .if eixsts(${_s:R}.m})