Index: share/mk/bsd.lib.mk
===================================================================
--- share/mk/bsd.lib.mk
+++ share/mk/bsd.lib.mk
@@ -255,6 +255,10 @@
 
 .if defined(LIB) && !empty(LIB) || defined(SHLIB_NAME)
 OBJS+=		${SRCS:N*.h:${OBJS_SRCS_FILTER:ts:}:S/$/.o/}
+.if ${OBJS:M/*}
+.warning "Found absolute path to .o file, this will almost certainly\
+    break the build when using a read-only source directory: ${OBJS:M/*}"
+.endif
 BCOBJS+=	${SRCS:N*.[hsS]:N*.asm:${OBJS_SRCS_FILTER:ts:}:S/$/.bco/g}
 LLOBJS+=	${SRCS:N*.[hsS]:N*.asm:${OBJS_SRCS_FILTER:ts:}:S/$/.llo/g}
 CLEANFILES+=	${OBJS} ${BCOBJS} ${LLOBJS} ${STATICOBJS}
Index: share/mk/bsd.prog.mk
===================================================================
--- share/mk/bsd.prog.mk
+++ share/mk/bsd.prog.mk
@@ -142,6 +142,10 @@
 .if defined(SRCS)
 
 OBJS+=  ${SRCS:N*.h:${OBJS_SRCS_FILTER:ts:}:S/$/.o/g}
+.if ${OBJS:M/*}
+.warning "Found absolute path to .o file, this will almost certainly\
+    break the build when using a read-only source directory: ${OBJS:M/*}"
+.endif
 
 # LLVM bitcode / textual IR representations of the program
 BCOBJS+=${SRCS:N*.[hsS]:N*.asm:${OBJS_SRCS_FILTER:ts:}:S/$/.bco/g}