package/exim: Fix compilation error with musl

Fixes
smtp_in.c: In function ‘smtp_start_session’:
smtp_in.c:1976:36: error: invalid application of ‘sizeof’ to incomplete type ‘struct options’
     EXIM_SOCKLEN_T optlen = sizeof(struct ip_options) + MAX_IPOPTLEN;

smtp_in.c misdetects the needed style for ip_options, only OPTSTYLE == 2 works.
Since musl does not provide any macro[1] to detect it we need to pretend to be
darwin in order to fix the compile bug.

[1] http://wiki.musl-libc.org/wiki/FAQ#Q:_why_is_there_no_MUSL_macro_.3F



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
[Luca: simplify by using only one sed invocation]
Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>