make codecheck fails for ign-cmake itself on 18.04

[scpeters]

With the recent change to use make codecheck instead of sh tools/code_check.sh in gazebo-tooling/action-gz-ci#26, the GitHub action for ign-cmake on 18.04 is now failing. I can reproduce the failure on 18.04 by running make codecheck on the ign-cmake2 branch:

$ make codecheck
Scanning dependencies of target cppcheck
cppcheck: argument to '-I' is missing.
examples/CMakeFiles/cppcheck.dir/build.make:57: recipe for target 'examples/CMakeFiles/cppcheck' failed
make[3]: *** [examples/CMakeFiles/cppcheck] Error 1
CMakeFiles/Makefile2:1675: recipe for target 'examples/CMakeFiles/cppcheck.dir/all' failed
make[2]: *** [examples/CMakeFiles/cppcheck.dir/all] Error 2
CMakeFiles/Makefile2:1575: recipe for target 'examples/CMakeFiles/codecheck.dir/rule' failed
make[1]: *** [examples/CMakeFiles/codecheck.dir/rule] Error 2
Makefile:730: recipe for target 'codecheck' failed
make: *** [codecheck] Error 2

$ make codecheck VERBOSE=1
...
cd /home/ignition/clone/ign-cmake/build/examples && /usr/bin/cppcheck -q --inline-suppr -j 4 --language=c++ --std=c++14 --force --enable=style,performance,portability,information -I -UM_PI\ --rule-file=/header_guard.rule\ --rule-file=/namespace_AZ.rule `/usr/bin/find -name '*.cc' -o -name '*.hh' -o -name '*.c' -o -name '*.h'`
cppcheck: argument to '-I' is missing.
...