compat: off64_t is not defined by musl

compat: off64_t is not defined by musl

This helps compile with latest musl, where off64_t is not defined unless
_LARGEFILE64_SOURCE is defined. On glibc, _LARGEFILE64_SOURCE is defined
if _GNU_SOURCE is defined, so the problem is only seen with musl.

Since the project uses AC_SYS_LARGEFILE, which from the autoconf doc:
"arrange for 64-bit file offsets, known as large-file support."

As such, it is safe to assume off_t is 64-bit wide. This is checked by a
static_assert to catch any platform where autoconf would let a 32-bit
off_t slip.

Reported-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: If2c6007a8c85bc3f3065002af8a7538b882fb4a8