I never heard of `.sx` but google lead to [this](http://www.nongnu.org/avr-libc/user-manual/assembler.html):
> As an alternative to using .S, the suffix .sx is recognized for this purpose > (starting with GCC 4.3.0). This is primarily meant to be compatible with > other compiler environments that have been providing this variant before in > order to cope with operating systems where filenames are case-insensitive > (and, with some versions of make that could not distinguish between .s and .S > on such systems). If it doesn't clash, I see no reason not to also add it. --- Reply to this email directly or view it on GitHub: https://github.com/geany/geany/issues/904#issuecomment-183864943
