Make the test harder by always having a space in the include dirs

The commit message for the previous commit was wrong, it should
have been: fix the test by using $(shell ...) syntax instead
of backticks in the Makefile.
With backticks I couldn't get the quoting right.
Printing -I"/some/path with space" did not work, the compiler
complained that there is not file "with". Also backslashes in
different numbers did not make it work.

Alex