net-mgmt/xymon-server: fix build with GCC-based architectures

Adding -L/usr/lib when using gcc8 (used on GCC architectures) makes gcc8 link to base libstdc++ which breaks build.

Fix this by adding -L${OPENSSLBASE} only when using non-base SSL.

PR: 237956
Approved by: feld (maintainer timeout), linimon (mentor, on IRC)