Fix mingw warnings with -U__STRICT_ANSI__.

We use some C99 features (like lrintf) despite passing
-std=c89 -pedantic. This works on normal linux and mac
builds because we define _GNU_SOURCE, which enables
them, but the mingw32 headers ignore this, generating
warnings for lrintf and _putenv, the later hacked in
by libtool.

Avoid these by undefining __STRICT_ANSI__ in mingw builds,
which allows the default extensions and fixes the warnings.