diff --git a/share/mk/src.sys.mk b/share/mk/src.sys.mk
index edf369638d9d..d5c2af0c559d 100644
--- a/share/mk/src.sys.mk
+++ b/share/mk/src.sys.mk
@@ -1,46 +1,49 @@
 
 # Note: This file is also duplicated in the sys/conf/kern.pre.mk so
 # it will always grab SRCCONF, even if it isn't being built in-tree
 # to preserve historical (and useful) behavior. Changes here need to
 # be reflected there so SRCCONF isn't included multiple times.
 
 .if !defined(_WITHOUT_SRCCONF)
 # Allow user to configure things that only effect src tree builds.
 SRCCONF?=	/etc/src.conf
 .if !empty(SRCCONF) && \
     (exists(${SRCCONF}) || ${SRCCONF} != "/etc/src.conf") && \
     !target(_srcconf_included_)
 
 # Validate that the user didn't try setting an env-only variable in
 # their src.conf. This benefits from already including bsd.mkopt.mk.
 .for var in ${__ENV_ONLY_OPTIONS:O:u}
 __presrcconf_${var}:=	${MK_${var}:U-}${WITHOUT_${var}:Uno:Dyes}${WITH_${var}:Uno:Dyes}
 .endfor
 
 .sinclude "${SRCCONF}"
 _srcconf_included_:	.NOTMAIN
 
 # Validate the env-only variables.
 .for var in ${__ENV_ONLY_OPTIONS:O:u}
 __postrcconf_${var}:=	${MK_${var}:U-}${WITHOUT_${var}:Uno:Dyes}${WITH_${var}:Uno:Dyes}
 .if ${__presrcconf_${var}} != ${__postrcconf_${var}}
 .error Option ${var} may only be defined in ${SRC_ENV_CONF}, environment, or make argument, not ${SRCCONF}.
 .endif
 .undef __presrcconf_${var}
 .undef __postrcconf_${var}
 .endfor
 
 .endif # SRCCONF
 .endif
 
 # The following should be removed no earlier than LLVM11 being imported into the
 # tree, to ensure we don't regress the build.  LLVM11 and GCC10 will switch the
 # default over to -fno-common, making this redundant.
 CFCOMMONFLAG?=	-fno-common
 CFLAGS+=	${CFCOMMONFLAG}
+.if defined(PACKAGE_BUILDING)
+CFLAGS+=	-fmacro-prefix-map=${SRCTOP}=/usr/src -fdebug-prefix-map=${SRCTOP}=/usr/src
+.endif
 
 DEFAULTWARNS=	6
 
 # tempting, but bsd.compiler.mk causes problems this early
 # probably need to remove dependence on bsd.own.mk 
 #.include "src.opts.mk"