flatc uses `strto<foo>_l()` over `strto<foo>()` when available, so that
they behave reproducible. The musl libc has no locale support, which
has the added benefit of reproducible behavior being the default.
For some reason a `strtof_l()` compatibility wrapper was added in
musl, but the other `strto<foo>_l()` wrappers are missing. This
adds a patch to check for `strtoull_l()` instead of `strtof_l()`. This
will not make a difference on a libc that supports all of them, but
fixes compilation on musl.
When building with make -j, flatbuffers is cloned by both the flatc build and the regular package build and from time to time a concurrency issue make the build to fail. Keeping regular flatbuffer package and flatc tool sources separate avoid this issue
