[J-core] musl-cross-make / litecross improvements

[Rich Felker]

I've made a number of improvements and fixes to musl-cross-make which
are in the upstream repo now, based on bug reports I got:

- Spurious dependency on flex (upstream's fault, worked around)
- Spurious dependency on texinfo (ditto)
- Ability to use pre-installed gmp/mpc/mpfr libs
- Config examples for static linking, no-debug builds.

https://github.com/richfelker/musl-cross-make

A few things I still plan to add at some point:

- Support for GCC 6, maybe older GCC's too
- Fixing configure bottlenecks with config.cache
- Getting "make check" working
- Automating multi-target builds
- Automating building binary static-linked-vs-musl toolchains

Let me know if any major bugs/inconveniences remain that I should
prioritize fixing.

Rich