Rename make variable LIBS

If the initial environment happens to contain a variable LIBS, then make will export its variable LIBS, overriding the environment (see e.g. <https://www.gnu.org/software/make/manual/html_node/Variables_002fRecursion.html>). This will break libuv/configure, since Julia's LIBS variable is not what autoconf expects in $LIBS. libuv/configure will then try to execute e.g. "cc -c conftest.c libuv", which is obviously wrong.

Renaming LIBS to DEPS_LIBS solves the issue.

This (and changes below it) is unrelated, right? Could you put them in separate commits? Thanks!