buildsys: test in CI that we use config.h correctly

dev/audit-config-h.sh

@@ -0,0 +1,23 @@
+#!/bin/sh
+#
+# This script verifies that we are not accidentally using config.h from one of
+# our header files.
+set -e
+
+# First scan config.h to obtain a list of all symbols it might define. From
+# this is subtracts a list of "known OK" symbols. The symbols and there reason
+# for being on this list are:
+# - `HAVE_FUNC_ATTRIBUTE_`*: these are only used for optimizations; also, our
+# headers try hard to define them on the fly (at least in GCC and clang)
+# - `HAVE___BUILTIN_MUL_OVERFLOW`: same as above
+# - `SIZEOF_VOID_P`: provided for backwards compatibility in a few packages,
+# and actually (re-)defined in `common.h`
+# - `SPARC`: only appears in a comment
+PATTERN=$(egrep '(#define|#undef)' build/config.h | sed -E -e 's;(#define|/\* #undef) ([^ ]+) .*$;\2;' | egrep -v 'HAVE_FUNC_ATTRIBUTE_|HAVE___BUILTIN_MUL_OVERFLOW|SIZEOF_VOID_P|SPARC' | tr '\n' '|')
+PATTERN=${PATTERN%?} # remove trailing "|"
+
+# Next use `git grep` to search all kernel header files for occurrences of any
+# of the symbols in the pattern we just created. We negate the exit code: grep
+# exits with exit code 0 if there were hits, and with exit code 1 if there
+# were no hits. For our purposes, we want the reverse: no hits is "good".
+! git grep -n -P ${PATTERN} :src/*.h

dev/ci.sh

@@ -159,6 +159,9 @@ GAPInput
  # test: running `make` a second time should produce no output
  test -z "$(make)"
 
+ # audit config.h
+ $SRCDIR/dev/audit-config-h.sh
+
  # test: touching all source files does *not* trigger a rebuild if we make
  # a target that doesn't depend on sources. We verify this by replacing the source
  # code with garbage