Make pgwin32_putenv() probe every known CRT, regardless of compiler.

Make pgwin32_putenv() probe every known CRT, regardless of compiler.

This extends to MinGW builds the provision for MSVC-built libraries to
see putenv() effects.  Doing so repairs, for example, the handling of
the krb_server_keyfile parameter when linked with MSVC-built MIT
Kerberos.  Like the previous commit, no back-patch.