Don't provide a default fma implementation

Define fma for integers and rational only.
Also define fma for BigFloat.
Merge branch 'fma'
Rename LIBS variable to avoid naming conflict

If the initial environment contains a variable LIBS, then the top-level Makefile exports it to deps/Makefile, which then makes libuv/configure fail.
Correct typo in comment
Merge branch 'LIBS'