Prefer external csnappy library

This patch checks for presence of csnappy library and header file and
if they are available, they will be used to link to the
Sereal::Encoder and Sereal::Decoder XS modules. Otherwise the bundled
csnappy code will be used.

<https://github.com/Sereal/Sereal/issues/72>

Signed-off-by: Petr Písař <ppisar@redhat.com>
Prefer external miniz library

This patch checks for presence of miniz library and header file and
if they are available, they will be used to link to the
Sereal::Encoder and Sereal::Decoder XS modules. Otherwise the bundled
miniz code will be used.

<https://github.com/Sereal/Sereal/issues/72>

Signed-off-by: Petr Písař <ppisar@redhat.com>
Allow one to override the detection of miniz/csnappy libraries
sync up with csnappy ca56f68aab
Release v3.002_002 - Build improvements and other minor changes