Workaround cppcheck ignoring -i libpromises/cf3lex.c

When running cppcheck in static checks, we use `-i
libpromises/cf3lex.c` in order to make cppcheck ignore the
generated cf3lex.c file. However, the new version of cppcheck
ignores this option and fails on issues found in the file.

However, we don't want this file or any other generated files
except for bootstrap.inc which contains the bootstrap policy. So
we can run `make clean` and `make -C libpromise/ boostrap.inc`
before running cppcheck to get a cleaner environment.

tests/static-check/run_checks.sh

@@ -21,6 +21,8 @@ function check_with_clang() {
 
 function check_with_cppcheck() {
   rm -f config.cache
+  make clean
+  make -C libpromises/ bootstrap.inc # needed by libpromises/bootstrap.c
   ./configure -C --enable-debug
 
   # cppcheck options: