Fix compilation warning by MinGW GCC.

 setenv.c [!HAVE_ENVIRON_DECL]: Avoid declaring environ if it is
 a macro, as this causes compiler warnings with MinGW.
Avoid MinGW warnings about redefinition of snprintf.

 gdb/python/python-internal.h (HAVE_SNPRINTF)
 [_WIN32 && HAVE_DECL_SNPRINTF]: Define, to avoid compiler warnings
 about redefinition of snprintf by pyerrors.h.