Xpra: Ticket #1910: per distro spec files

Usually, we don't update spec files in older branches, only in trunk. The problem with this is that some distros (especially centos6 at the moment) are no longer supported by newer versions of some packages and so the updates make it impossible to rebuild all the dependencies from one tag, or even from trunk.

We should lock those spec files at the last revision that does build. ie: rpmbuild/centos6/ would contain the centos6 spec files locked at:

• x264 pre 2018 (newer requires newer nasm)
• pycuda ~2015?
• python-lz4 - who knows (constant breakage)

etc

And maybe also prevent our spec files from building:

• libraries already available from upstream (ie: pygtkglext is available in centos6)
• libraries we know cannot be built and we don't care (ie: python-uinput not useful for centos6)