A new binary object which tests linking with libfontforge.la

This is a first step towards #465 and #469.

I added a new near-empty c file, which links to `libfontforge.la`. The file is in the `tests` folder, which will not be installed.

This target is a normal target in `tests`, which is always compiled, not only when `make check` is called.(Sorry about the commit message) I made this because actually I don't know how to make it that way. Or shall I write a new script in the testsuite? That way it would be harder to handle compilers, flags, and paths -- but possible.

Since it is a normal target, if this pull request is merged, `make` will fail, so #465 should be fixed beforehand.

Also please check if any FLAGS should be added for this target.

BTW, why there are xxx_CPPFLAGS used in many makefiles? I don't think we have CPP files.