package/softether: fix static build with openssl and atomic

Don't pass --with-openssl option to force softether to use pkg-config
(see autotools/ax_check_openssl.m4).

pkg-config will find openssl dependencies such as lz or latomic

Fix static build on sparc v8 (even if there is no autobuilder failures
yet)

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>