Suppress compiler warnings in ecpg test on newer Windows toolchains.

nan_test.pgc supposed that it could unconditionally #define isnan()
and isinf() on WIN32.  This was evidently copied at some point from
src/include/port/win32.h, but nowadays there's a test on _MSC_VER
there.  Make nan_test.pgc look the same.

Per buildfarm warnings.  There's no evidence this produces anything
worse than a warning, and besides it's only a test case, so I don't
feel a need to back-patch.