This fixes a nasty bug that manifests on windows when building without
precompiled headers. util/stringops.h used to silently replace strdup
with a macro that's compatible with mem_free(). This header would
typically be included everywhere due to PCH, but without it the strdup
from libc would sometimes be in scope. On most platforms mem_free() is
equivalent to free(), but not on windows, because we have to use
_aligned_free() there. Attempting to mem_free() the result of a libc
strdup() would segfault in such a configuration.
Avoid the footgun by banning strdup() entirely. Maybe redefining libc
names isn't such a great idea, who knew?
