Makefile: Avoid using built-in stpcpy during clang build

Makefile: Avoid using built-in stpcpy during clang build

Since clang 12, during the clang build, noticed by the CI, the linking
fails as clang optimize some string functions to stpcpy.

  LINK fio
lld-link: error: undefined symbol: stpcpy
>>> referenced by C:\projects\fio\options.c:5305
>>>               options.o:(fio_options_parse)

Two possible implementations :
- Adding stpcpy in fio as the kernel did : https://lore.kernel.org/lkml/20200815002417.1512973-1-ndesaulniers@google.com/T/
- Disable the implicit stpcpy

To avoid adding code into fio, the latter option was used.

Signed-off-by: Erwan Velu <e.velu@criteo.com>