meson: fix usage as a subproject
This should be enough to get it into WrapDB.
One interesting issue I noticed is that if a projects is built as say, C99, strdup being implicit fails the build in testrecurse.c and runtest.c. I threw _GNU_SOURCE defines at the files to get it to build but still interesting. I see in gitlab-ci.yml there's a -D_XOPEN_SOURCE=600 define. I assume to add strdup.