From 892d21b17164713f7eb655d340be7c77f4b5104e Mon Sep 17 00:00:00 2001
From: David Lamparter <equinox@opensourcerouting.org>
Date: Tue, 21 Aug 2018 20:33:13 +0200
Subject: [PATCH] build: add warning when overwriting vars on "make"

doing things like `make CC="mmix-linux-musl-gcc"` breaks the hosttools/
cross-compilation setup pretty hard and just straight up should not be
done.  These vars belong on `configure`, not `make`.

Signed-off-by: David Lamparter <equinox@diac24.net>
---
 Makefile.am  | 42 ++++++++++++++++++++++++++++++++++++++++++
 configure.ac |  4 +++-
 2 files changed, 45 insertions(+), 1 deletion(-)

diff --git a/Makefile.am b/Makefile.am
index ec186b3fc1..0ecab8b1f9 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -8,6 +8,48 @@ AM_CPPFLAGS += -I$(top_srcdir) -I$(top_srcdir)/include -I$(top_srcdir)/lib \
 DEFS = @DEFS@ -DSYSCONFDIR=\"$(sysconfdir)/\" -DCONFDATE=$(CONFDATE)
 LIBCAP = @LIBCAP@
 
+# overwriting these vars breaks cross-compilation.  let's be helpful and warn.
+#
+# note: "#AUTODERP# " will be removed from Makefile by configure.  These are
+# GNU make directives & automake will f*ck them up by trying to process them
+# as automake directives.
+#
+#AUTODERP# null=
+#AUTODERP# SPACE=$(null) $(null)
+#AUTODERP# mkcheck_CC =       $(findstring $(SPACE)CC=,      $(SPACE)$(MAKEOVERRIDES))
+#AUTODERP# mkcheck_CFLAGS =   $(findstring $(SPACE)CFLAGS=,  $(SPACE)$(MAKEOVERRIDES))
+#AUTODERP# mkcheck_CPPFLAGS = $(findstring $(SPACE)CPPFLAGS=,$(SPACE)$(MAKEOVERRIDES))
+#AUTODERP# mkcheck_CCLD =     $(findstring $(SPACE)CCLD=,    $(SPACE)$(MAKEOVERRIDES))
+#AUTODERP# mkcheck_LD =       $(findstring $(SPACE)LD=,      $(SPACE)$(MAKEOVERRIDES))
+#AUTODERP# mkcheck_LDFLAGS =  $(findstring $(SPACE)LDFLAGS=, $(SPACE)$(MAKEOVERRIDES))
+#AUTODERP# #
+#AUTODERP# ifneq ($(mkcheck_CC),)
+#AUTODERP#   $(warning WARNING: you have overwritten the "CC" variable on the make command line.)
+#AUTODERP# endif
+#AUTODERP# ifneq ($(mkcheck_CFLAGS),)
+#AUTODERP#   $(warning WARNING: you have overwritten the "CFLAGS" variable on the make command line.)
+#AUTODERP# endif
+#AUTODERP# ifneq ($(mkcheck_CPPFLAGS),)
+#AUTODERP#   $(warning WARNING: you have overwritten the "CPPFLAGS" variable on the make command line.)
+#AUTODERP# endif
+#AUTODERP# ifneq ($(mkcheck_CCLD),)
+#AUTODERP#   $(warning WARNING: you have overwritten the "CCLD" variable on the make command line.)
+#AUTODERP# endif
+#AUTODERP# ifneq ($(mkcheck_LD),)
+#AUTODERP#   $(warning WARNING: you have overwritten the "LD" variable on the make command line.)
+#AUTODERP# endif
+#AUTODERP# ifneq ($(mkcheck_LDFLAGS),)
+#AUTODERP#   $(warning WARNING: you have overwritten the "LDFLAGS" variable on the make command line.)
+#AUTODERP# endif
+#AUTODERP# #
+#AUTODERP# ifneq ($(mkcheck_CC)$(mkcheck_CFLAGS)$(mkcheck_CPPFLAGS)$(mkcheck_CCLD)$(mkcheck_LD)$(mkcheck_LDFLAGS),)
+#AUTODERP#   $(warning ------)
+#AUTODERP#   $(warning While overwriting these variables works most of the time, it is not recommended and can cause confusing build errors.)
+#AUTODERP#   $(warning This is especially problematic when cross-compiling, since tools that run on the build system during the build process will not be compiled correctly.)
+#AUTODERP#   $(warning All of these variables should be supplied to 'configure', and they will be remembered and correctly applied during 'make'.)
+#AUTODERP#   $(warning ------)
+#AUTODERP# endif
+
 EXTRA_DIST =
 BUILT_SOURCES =
 CLEANFILES =
diff --git a/configure.ac b/configure.ac
index 2d8723d5cf..12c1857758 100755
--- a/configure.ac
+++ b/configure.ac
@@ -1993,7 +1993,9 @@ AC_CACHE_VAL(ac_cv_htonl_works,
 )
 AC_MSG_RESULT($ac_cv_htonl_works)
 
-AC_CONFIG_FILES([Makefile
+AC_CONFIG_FILES([Makefile],[sed -e 's/^#AUTODERP# //' -i Makefile])
+
+AC_CONFIG_FILES([
 	  bgpd/Makefile
 	  vtysh/Makefile
 	  tests/Makefile
-- 
2.39.5