Index: lib/libc/sys/Makefile.inc
===================================================================
--- lib/libc/sys/Makefile.inc
+++ lib/libc/sys/Makefile.inc
@@ -49,7 +49,7 @@
 SRCS+= pipe.c
 SRCS+= shm_open.c
 SRCS+= vadvise.c
-SRCS+= _Fork.c
+SRCS+= POSIX2x_Fork.c
 
 SRCS+=	compat-stub.c
 
@@ -120,6 +120,13 @@
 
 SYM_MAPS+=	${LIBC_SRCTOP}/sys/Symbol.map
 
+# Work around an issue on case-insensitive file systems.
+# libc has both _Fork.c and _fork.s and they both yield
+# _fork.o (case insensitively speaking).
+CLEANFILES+=POSIX2x_Fork.c
+POSIX2x_Fork.c: ${LIBC_SRCTOP}/sys/_Fork.c .NOMETA
+	ln -sf ${.ALLSRC} ${.TARGET}
+
 # Generated files
 CLEANFILES+=	${SASM} ${SPSEUDO}