Remove warning message when building fpm

[wyphan]

Description

This is not a game-breaking bug by any means, but it would be nice if this warning message when building fpm no longer shows up at the link phase.

/usr/bin/ld: build/gfortran_367ECB55A902040F/fpm/libfpm.a(src_fpm_filesystem.F90.o): in function `__fpm_filesystem_MOD_get_temp_filename':
/fpm/./src/fpm_filesystem.F90:591: warning: the use of `tempnam' is dangerous, better use `mkstemp'

Expected Behaviour

Building fpm does not emit any warning messages.

Version of fpm

It's been there for a while, incuding latest develop (at the time of writing) aec789f

Platform and Architecture

Linux x86_64 (Ubuntu 20.04 LTS)

Additional Information

No response