fix: build failure when _SC_PHYS_PAGES is not defined

_SC_PHYS_PAGES isn't POSIX and might not be defined. in such case, just
return back `CACHE_SIZE_FALLBACK`.

NOTE: POSIX says the `names` in `sysconf()` are "symbolic constants" not
necessarily macros. So we might end up returning the fallback in some
cases where `_SC_PHYS_PAGES` *was* available, but not defined as a
macro. which is not ideal, but nothing fatal.

in practice, this shouldn't be an issue since most systems seems to
define them to be macros, i've checked Glibc, Musl, OpenBSD, FreeBSD and
Haiku.

also add a (useful) comment on `config.h` describing the effect higher
cache size has.

Closes: https://codeberg.org/nsxiv/nsxiv/issues/354

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