Work around gcc issues with limits.h not defining LLONG_MIN

It looks like there are several cases where this might happen:

- In some MinGW distributions, the LLONG_MIN/etc defines are guarded
with:

	#if !defined(__STRICT_ANSI__) && defined(__GNUC__)

Which means that you don't get them in strict ANSI mode. The previous
workaround was specifically targeted towards this.

- In some GCC distributions (notably GCC 6.3.0 in some configurations),
LLONG_MIN/etc. defines are guarded with:

	#if (defined (__STDC_VERSION__) && __STDC_VERSION__ >= 199901L)

But __STDC_VERSION__ isn't defined as C99 even if you use -std=c++14 -
which is probably technically valid, but not useful.

To work around this, redefine the symbols whenever we are building with
GCC and we need them and they aren't defined - doing this is better than
not building. Instead of hard-coding the constants, use GCC-specific
__LONG_LONG_MAX__ to compute them.

Fixes #181.