MINOR: flt-decomp: warn if built without USE_SLZ

MINOR: flt-decomp: warn if built without USE_SLZ

For now we only support some decompression algos by leveraging libslz
builtin library. But it can be confusing for users building haproxy
against zlib (with USE_ZLIB). For such cases, we now raise an explicit
error telling that the algos are not available because they require
haproxy to be built using USE_SLZ.