rescript-tools: do not override flags for static linking

[cknitt]

Fixes #1006.

[woeps]

I ran DUNE_PROFILE=static opam exec -- make locally on my linux machine and calling file ./_build/default/tools/bin/main.exe now gives:

main.exe: ELF 64-bit LSB executable, x86-64, version 1 (GNU/Linux), statically linked, BuildID[sha1]=605ca79a225725810f29e03d1df972cb0abaa4d8, for GNU/Linux 4.4.0, with debug_info, not stripped

I assume running the same command in ci (using alpine image) should have similar results.

Thanks for your efforts!

[zth]

@woeps releasing this in 0.6.3.