security/git-crypt: fix build with GCC-based architectures

Add USES=compiler:c++11-lang.

Don't add -L${OPENSSLLIB} unconditionally, do it only when SSL library from ports is used. The reason is that adding -L/usr/lib makes ports GCC want to link to libstdc++ from /usr/lib, not from /usr/local/lib/gcc8. This causes linking errors

PR: 237525
Approved by: ashish (maintainer), tcberner (mentor)
Differential Revision: https://reviews.freebsd.org/D20053