[REQUEST] Make it possible to have a subset of externally built libraries for Blosc in FindBlosc.cmake

[vasil-pashov]

Is your feature request related to a problem? Please describe.

Currently, there is an option BLOSC_USE_EXTERNAL_SOURCES in FindBlosc.cmake which controls whether Blosc was built with external sources. This option however is all or nothing. It suggests that Blosc is built either with the in-source version of all libraries (lz4 snappy zlib zstd) or all of them are using custom sources.

Describe the solution you'd like

Add parameter e.g. BLOSC_EXTERNAL_SOURCES_LIST allowing to pass only a subset of (lz4 snappy zlib zstd) used as an external source to build blosc.

Describe alternatives you've considered

I couldn't think of a workaround so I'm building Blosc using the in-source version it provides.