GOG.com resumed selling Descent 1 and Descent 2, albeit as separate SKUs
rather than the historical bundled SKU. Restore the GOG reference in
INSTALL.markdown.
Reported-by: derhass <https://forum.dxx-rebirth.com/showthread.php?tid=1004>
Using <gcc-4.9 has been unsupported since introduction of `extern
constexpr` variables broke using gcc-4.8. Update the install
instructions accordingly.
When referencing 4.9, qualify it as 4.9.4 due to gcc bug #66501 [1]
(present in 4.9.2, fixed in 4.9.4). As discussed in "Socket error upon
hosting or joining a second multiplayer game"
<https://github.com/dxx-rebirth/dxx-rebirth/issues/289>, this bug caused
a strange miscompilation in Rebirth. Although that specific
miscompilation has been worked around, there is no guarantee that the
code is and will remain free of other constructs that gcc-4.9.2
mishandles due to that bug. Using 4.9.2 (as Debian Jessie did) mostly
works, but there is no sense encouraging users to use a known buggy
compiler when a fixed version is readily available.
[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66501
