Update INSTALL with solution to SDL1 and SDL2 clash

[ryfactor]

Build system bugs if you have SDL1 and SDL2 installed at the same time. See: https://attnam.com/posts/29409 for the type of error and https://attnam.com/posts/29417 for the workaround

Probably needs fixing at the cmake level

[AquariusPower]

what about dropping/deleting all the code related to DJGPP and SDL1 ?
has anyone tested any in the last 5 years ?
EDIT: by what I read, dropping the untested code will not fix SDL1 conflict tho.

[jakwings]

This may help: export SDL2DIR=/path/to/include/SDL2;/path/to/lib

EDIT: It seems the lexigraphical sort order "SDL < SDL2" is the problem.