Clean up zlib's build script

We should clean up the compile and link flags zlib build script:

1. CFLAGS and LDFLAGS seem to be unused, rather LOC is taken to define those.
2. Given 1) we could just assign the relevant flags to LOC and omit CFLAGS and LDFLAGS entirely AND we can omit spelling LOC out twice (once in the make command and once in the make install command).
3. While we are at it, just doing a make install might be enough.
4. No need to hard-code -j4.