Build with bundled libstddjb selfpipe by default

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

third/libstddjb/Makefile.am

@@ -0,0 +1,35 @@
+AM_CFLAGS = -pedantic -Wall -Wextra -pipe
+
+noinst_LIBRARIES = libstddjb.a
+
+libstddjb_a_SOURCES = \
+	allreadwrite.h \
+	coe.c \
+	djbunix.h \
+	error.h \
+	fd_close.c \
+	fd_read.c \
+	fd_write.c \
+	gccattributes.h \
+	ndelay_on.c \
+	nsig.h \
+	pipe_internal.c \
+	sanitize_read.c \
+	selfpipe-internal.h \
+	selfpipe.h \
+	selfpipe_finish.c \
+	selfpipe_init.c \
+	selfpipe_internal.c \
+	selfpipe_read.c \
+	selfpipe_trap.c \
+	selfpipe_trapset.c \
+	selfpipe_untrap.c \
+	sig.h \
+	sig_restoreto.c \
+	skalibs-config.h \
+	skasig_dfl.c \
+	skasigaction.c \
+	sysdeps.h \
+	uint64.h
+
+EXTRA_DIST = COPYING