Resolves build issue on Arch linux.
As noted in the attached trac tickets, setting the environment variable SDL2_CONFIG to pkg-config sdl2 appears to work as a work-around to the issue reported in both the tickets and on the forums (https://wildfiregames.com/forum/index.php?/topic/24180-icu-and-stdlib-error-svn/). (It is also the solution/work-around used by the Arch package maintainer.)
Looking into why it works, we can see the outputs of both sdl2-config --cflags (which is used if the environmental variable is not set) and the work-around:
$> sdl2-config --cflags -I/usr/include/SDL2 -I/usr/include -D_REENTRANT $> pkg-config sdl2 --cflags -D_REENTRANT -I/usr/include/SDL2
The missing /usr/include from the second output (the absence of which makes the compile work), is a "standard system include" and thus is implicitly included in the includes-search-list. When sdl2-config explicitly includes it, it appears to be added to the includes-search-list too early, thus causing an issue with gcc's #include_next macro.